It's not a secret that the Gas Passer is incredibly controversial within the MvM community. From my own experience I have seen variations of players who swear by the Gas Passer, those who avoid it like the plague, and some who are completely apathetic to it regardless of whether they use it themselves or one of their team does.

Disregarding my own opinion, in this guide I will be attempting to analyse the Gas Passers nature in MvM and determining exactly why it is so controversial. Whether or not it deserves all the hate and praise it gets is up to you to decide for yourselves.

Before delving into the nature of its controversy, I feel it's necessary to first dissect this weapon and lay out all the facts plain and simple. In this section I will be listing all the information I can gather about the Gas Passer, including its mechanics, stats, upgrades, bugs etc.



Stats - Upsides

• When thrown the impacted area is covered in a cloud of gas. Robots within the gas cloud at any point of it's duration will be coated in gas (this appears as green, orange and red particles of liquid on the robots model). Upon taking any form of damage while coated in this gas, the robot will be ignited and take afterburn damage.
  
• The afterburn damage deals 4 damage per tick (8 damage per second).
  
• The recharge meter on this weapon is charged over time and through dealing damage. It takes 60 seconds to reach full charge with no damage dealt, or 750 damage to reach full charge with no time interval.
  
• Upon death any charge is retained after respawning (does not charge passively while dead).
  
• Can be used to extinguish teammates (only the initial impact has this effect).

Stats - Downsides

• Resupply and spawning do not max the charge of the Gas Passer.
  
• Upon first using the Gas Passer within a session as Pyro (via wave failure, switching classes, or first joining a game), the charge will be empty.

Upgrades

• Explode On Ignite - Enemies who are coated in gas will explode for 350 damage upon being ignited.
  
• 15% Recharge Rate #1 - This reduces the recharge rate to 51 seconds or 637.5 damage.
  
• 15% Recharge Rate #2 - This reduces the recharge rate to 42 seconds or 525 damage.
  
• 15% Recharge Rate #3 - This reduces the recharge rate to 33 seconds or 412.5 damage.
  
• 15% Recharge Rate #4 - This reduces the recharge rate to 24 seconds or 300 damage.

Mechanics

• The gas cloud on the impacted area lasts 5 seconds.
  
• Gas applied to robots lasts 10 seconds (5 seconds if the robot is constantly healed).
  
• Afterburn damage applied by the gas lasts 10 seconds (resulting in 80 damage in total).
  
• Explosions from the Explode On Ignite upgrade can trigger ignites on other coated enemies, resulting in chain explosions. These explosions chain all at once and are only limited by how many enemies are coated in gas.
  
• Robots can each only be affected by one instance of Explode On Ignite explosion damage at once (EG: if 5 Heavies explode around a giant Heavy at the same time, the giant Heavy will still only take 350 damage).
  
• The can itself has a maximum airtime of approximately 5 seconds before it bursts in midair, causing the gas cloud to linger wherever the can burst (gas formed in mid air will slowly move downwards).
  
• Its throwing distance is 935 hammer units (4.91 metres or 16.1 feet) when thrown at a straight angle (0 degree deviation from standard viewing angle) on a flat surface (no Y axis degree deviation from start to end of throw).
  
• Upon being reflected by a robot Pyro the Explode On Ignite upgrade will not take effect on RED players (this could be a bug, although it seems to be intentional as the same principle can be applied to reflected Mad Milk or Jarate, as the slowdown upgrade isn't applied to RED players).
  
• The gas cloud forms in a circular shape with a diameter of approximately 269 hammer units (5.25 meters).

Bugs

• The explosions from the Explode On Ignite upgrade do not have a tagged damage type, nor are they classified as ranged or melee damage, meaning it deals the full 350 damage to enemies like Steel Gauntlets and Demoknights (although generic all damage resistances still work on it).
  
• It is possible for the Gas Passers explosion from Explode On Ignite to cause self damage to the Pyro who threw it. If a robot coated in gas takes self damage (grenades, rockets, etc. at close range) or neutral damage (environmental or fall damage), the resulting explosion will damage the Pyro as well as any other nearby robots.
  
• The bug listed above can also be triggered if a Scout using the Shortstop shoves a robot coated in gas. The reason for this is because the Shortstops alt-fire also deals 1 neutral damage as well as knocking the target back.

In this section I will be attempting to analyse every point which could be considered good about the Gas Passer. For some of these points I will be comparing it to other classes with similar robot destruction capability. These are points which could be used to argue the point that the Gas Passer is simply too good.

Explode On Ignite - Robot Destruction Capability
Due to the combination of the Gas Passers explosion dealing more damage than the health of a Heavy and it being able to chain explosions off of each other, the Gas Passer can simply delete entire groups of robots with little to no effort. This can make some potentially challenging moments, which would normally require co-ordination or preparation (medic shields, demo traps, sniper focus saving), trivial as the Gas Passer can simply be thrown and triggered at any point.

Explode On Ignite - Medic Snuffing
Due to the reason mentioned above, the Explode On Ignite upgrade can essentially make dealing with Über or Quick Fix (small) Medics trivial. As in most MvM scenarios involving Medics, one of the team will have to prepare with classes capable of one-shotting Medics. A few examples of this are as follows:

• Demoman - Preparing sticky traps at appropriate locations potentially in combination with crit canteens or a Medics Kritzkrieg.
  
• Sniper - Finding an optimal position with easy sightlines to the medic target to allow for explosive headshot, and saving focus on the Hitmans Heatmaker.
  
• Soldier - Having appropriate damage upgrades to be able to one-shot the medics or purchasing crit canteens.
  
• Spy - Purchasing enough Robot Sapper Power upgrades to be able to stun all of the Medics allowing teammates to kill them with no risk of ÜberCharge usage.

Explode On Ignite - Timing
Although this is technically part of "Explode On Ignite - Medic Snuffing", I feel it's appropriate to give it its own category. When facing a target with Uber Medics, it is imperative that the team is informed, or is aware not to shoot the Uber Medics before your Medic picker is able to one-shot them with no risk of ÜberCharge usage. Shooting the medics before hand can cause them to use their ÜberCharge and can potentially be very punishing. Due to the nature of Explode On ignite detonating on any form of damage, this makes the punishment for shooting Über Medics early non-existent, as damage is needed to trigger the explosion, and will result in one-shots (as long as the targeted medics are coated in gas).

Cost - Comparison
Simply put, the Gas Passer with Explode On Ignite is the cheapest form of small robot destruction out of them all, requiring very few credits to be effective, and comparatively is the cheapest to optimise. Below I will list other classes with similar small robot destruction capability along with their minimum and optimal prices (this will not include survivability upgrades such as resistances).

• Pyro [Phlogistinator, Gas Passer] - 400 minimum, 3000 optimal
  
  • Minimum - Explode On Ignite [Gas Passer - 400 credits]
    
  • Optimal - +25% Damage x4 [Phlogistinator - 1600 credits], Explode On Ignite [Gas Passer - 400 credits], 15% Recharge Rate x4 [Gas Passer - 1000 credits]
• Sniper [Hitmans Heatmaker] - 1100 minimum, 4600 optimal
  
  • Minimum - Explosive Headshot [Hitmans Heatmaker - 350 credits], +20% Reload Speed x3 [Hitmans Heatmaker - 750 credits]
    
  • Optimal - +25% Damage x4 [Hitmans Heatmaker - 1600 credits], Projectile Penetration [Hitmans Heatmaker - 400 credits], Explosive Headshot x3 [Hitmans Heatmaker - 1050 credits], +20% Reload Speed x3 [Hitmans Heatmaker - 750 credits], 25% Faster Charge [Hitmans Heatmaker - 800 credits]
• Demo [Scottish Resistance] - 950 minimum, 3950 optimal
  
  • Minimum - +20% Reload Speed [Scottish Resistance - 750 credits], +10% Firing Speed [Scottish Resistance - 200 credits]
    
  • Optimal - +20% Damage x4 [Scottish Resistance - 2000 credits], +50% Clip Size x2 [Scottish Resistance - 800 credits], +20% Reload Speed [Scottish Resistance - 750 credits], +10% Firing Speed x2 [Scottish Resistance - 400 credits]
• Soldier [Beggars Bazooka] - 750 minimum, 5050 optimal
  
  • Minimum - +20% Reload Speed x3 [Beggars Bazooka - 750 credits]
    
  • Optimal - +25% Damage x4 [Beggars Bazooka - 1600 credits], +2 clip size x4 [Beggars Bazooka - 1600 credits], Rocket Specialist [Beggars Bazooka - 300 credits], +20% Reload Speed x3 [Beggars Bazooka - 750 credits], +10% Firing Speed x4 [Beggars Bazooka - 800 credits]

Cost - Early Game + Requirements
Although the cost differences between other similar loadouts and Gas Pyro doesn't seem that different, I want to clarify that the gas passer has the strongest minimum requirement. No other class is capable of dealing 350 damage in a large area at the cost of only 400 credits without the added assistance of either crit canteens or a Medics Kritzkrieg, even when comparing it to a Sniper, who is known to have incredibly high group destruction capability. A sniper with only 400 credits is only able to deal 150 damage to a group of robots when using explosive headshot, which requires snipers optimal positioning and aim. Whereas Gas deals 350 damage in a larger area due to the explosives chain effect and small area of effect on each explosion, and has no timing, positioning or otherwise specified requirements (aside from having the upgrade and hitting a group of robots with the gas cloud).

Recharge - Multiple Reduction Methods
Due to the fact that the recharge meter on this weapon is directly tied to damage output, there are multiple ways to reduce the amount of time in combat it takes to fully recharge this weapon. These are as follows:

• Upgrading +25% damage [any flame thrower - x1 to x4] - Upon upgrading damage on any flamethrower, direct flame damage is increased, which is Pyros biggest source of damage, and heavily impacts how quickly Gas recharges, especially when used against high groups of robots
  
• Upgrading +15% recharge rate [Gas Passer - x1 to x4] - As mentioned in The Facts section, the charge reduction upgrade doesn't just reduce the time it takes for the gas passers meter to recharge. It also reduces the overall damage requirement to fill the meter which start at 750 with no upgrades, and is reduced to 300 damage with all four of the upgrades.
  
• Using the Phlogistinator - The Phlogs main upside is that upon dealing 1200 damage against robots (3000 against tanks) it can be alt-fired to gain crits for 10 seconds. While the Gas Passers explosion damage does not charge the Phlog it still works the other way round, and the extra damage the crits from the Phlog provide can be used to charge the Gas Passers meter very quickly.
  
• Using crit canteens - Basically the same effect as using the Phlogs alternative fire, except there is no wind up time. The alt fire on the Phlog requires taunting unless the player specifically cancels the taunt by standing on top of a robots model or falling off a moving tank while taunting.

Critical Hits and Mini-crits
Critical Hits with the Gas Passer have no impact on damage and only apply a visual effect in the form of a team coloured glow on the projectile, however mini-crits do effect damage. It's explosion and afterburn effects will mini-crit as long as the Pyro is under the influence of a mini-crit boosting effect, or if the target is effected by a mini-crit debuff. The Gas Passer can mini-crit through the following ways:

• Buff Banner - All explosions and afterburn, as long as the Pyro is under its effect
  
• Jarate - Explosions and afterburn, only if the robots are coated in Jarate
  
• Marked for Death - Explosions and afterburn, only if the robot is marked for death (Sandman, Fan O'War, robot variation of the GRU)

Note - mini-crit explosions deal 457 damage, the afterburn effect deals 5 damage per tick.

Similar to the previous section, I will be analysing every point which could be considered a negative regarding the Gas Passer. Although it is an unbelievably good weapon as described in the last section it does have a few notable negatives. These are as follows.

Giant Damage
Unlike other classes with similar damage capacity when destroying smaller groups of robots, the gas passer has effectively no giant damage capacity, as only a single explosion can trigger on a giant at a time. While this can be impactful when throwing the gas repeatedly through primary damage recharging, it is arguably more effective to just use the Pyros primary against giants alone, since the switch and throw time are effectively DPS losses. Below I will list a few classes with much higher giant damage capacity:

• Demoman w/ Scottish Resistance - Up to 620 per stickybomb, potential for 14 stickybombs at once [with crit canteens or Kritzkrieg charge]
  
• Soldier w/ Beggars Bazooka - Up to 540 damage per rocket, potential for 11 rockets in rapid succession [with crit canteens or Kritzkrieg charge]
  
• Heavy w/ Brass Beast - 130 damage per shot at point blank [with crit canteens or Kritzkrieg charge]
  
• Sniper w/ Hitmans Heatmaker - 300 damage per headshot (900 fully charged), can use focus + max reload speed for rapid headshots
  
• Spy w/ Any Knife - 900 damage per backstab (aggro is pulled after 3 backstabs, if a medic is healing the spy, the aggro will transfer to the medic instead)

Lack of Mobility
A large amount of Pyros mobility is given from the secondary slot. While using gas the vertical mobility of the Pyro is severely limited. This is most noticeable on maps like decoy where it may be required for the Pyro to move from ground level to elevated platforms in quick succession. There are two better alternative secondaries for mobility, these are as follows:

• Detonator - When shot at the ground and crouch-jumping at the same time this greatly increases the Pyros vertical mobility.
  
• Thermal Thruster - Although this has a much longer deploy and holster time, the vertical and horizontal mobility and speed this provides is much more effective than the detonator, and requires no self damage to use. (aside from fall damage)

Note - Both of these mobility methods can also be used to get on top of tanks or robots (giant or small). If the Pyro lands on a robot and starts the taunt or falls off a tank at any point during the Phlog crits taunt it is cancelled, the duration of the crits effect is then extended. (this also gives a few seconds of damage immunity as it would normally during the taunt)

No Health Return
As the gas passer has no Health On Kill upgrade, any robots killed by the explosions or afterburn from this weapon return no health to the Pyro, even if the initial ignition was caused by said Pyros primary weapon with health on kill applied to it.
Note - If the robots are coated in Mad Milk, 60% of the damage from the explosions will return as healing (afterburn does return as mad milk healing)

In this section I will be detailing all valid criticisms I know of regarding the Gas Passer which do not fit within "The Good" or "The Bad". Some of these can be deemed as negative or positive depending on personal opinion.

Multi-tasking (Tank Shredding and Robot Destruction)
While attacking tanks the charge rate of the gas passer is not adjusted and still takes the same amount of damage to charge as it would when attacking robots. This might sound like a minor problem on paper, but in reality this can allow the Pyro to destroy tanks as he normally would, while throwing many Gas Passers at high traffic choke points. Tanks are easy to hit, high health targets, which means the Pyro has an easy source of near instant recharge for the Gas Passer with little effort. Combining this near instant recharge with Explode On Ignite and a reachable choke point (to throw gas at) bumps the Pyros damage capacity to even higher levels than it already is when using Gas.
Note - The Phlogistinator's Mmmph bar normally requires 300 damage to fill, which is increased in MvM to 1200 when damaging robots and 3000 when damaging tanks. While the Gas passer only requires 750 while damaging anything, reduced to 300 with full upgrades.

Bypassing Resistances
As mentioned in "The Facts" section under bugs, the Gas Passers explosions do not have a tagged damage type, nor are they tagged as ranged or melee damage. This essentially means that the Gas Passer deals the full 350 damage on explosion to all robots which do not have generic all damage reduction or immunity. Below I have listed all robots, I know of, which take full damage from the explosions unintentionally, based on the assumption that the explosion should be tagged as either fire or blast ranged damage, along with how much damage they theoretically should take:

• Steel Gauntlets (high health Fists of Steel Heavies) - Fists of Steel have the stat "-40% damage from ranged sources while active" so these robots should only take 210 damage.
  
• Demoknights + Giant Demoknights (Chargin' Targe variety) - Chargin' Targe has the stats "+50% fire resistance on wearer" and "+30% explosive resistance on wearer" so these robots should only take either 175 damage or 245 damage.
  
• Demoknights (Splendid Screen variety) - Splendid Screen has the stats "+20% fire damage resistance on wearer" and "+20% explosive damage resistance on wearer" so these robots should only take 280 damage.

Although generic all damage resistances and immunities still work on the Gas Passers explosions, I have listed these resistances and immunities which are used by certain robots below:

• Stock Medigun - Immune to all damage except knockback.
  
• Bonk! Atomic Punch - Immune to all damage except knockback. (damage is converted to a stacking slowdown effect)
  
• Batallions Backup - 35% damage reduction , 50% reduction to all sentry gun damage, and immunity to critical hits for all nearby robots (including the robot using the banner).

Uber Snuffing - The Alternative
Ever since the Gas Passers release, it has been a widespread opinion that it's explosions are the only way for Pyro to kill with Über medics with no risk of them using their ÜberCharge. While it is not common in the meta that Pyro plays the role of Medic picker, there is an alternative way of dealing with these Medics as a Pyro, which requires a source of crits (Kritzkrieg or crit canteen) but no upgrade investment. The Third Degree has only one stat and it's an upside as follows "All players connected via Medigun beams are hit". This essentially has no limit, and is capable of hitting all Medics healing a single target for 100% of the damage dealt by the initial hit. A standard hit with the Third Degree does 65 damage, but a critical hit does 195, which is able to one shot any standard Über or Quick-Fix Medic (excluding giant medics).

And that's all she wrote, so why exactly did I write this guide? Well, I found that loads of people say "no gas" or "gas op" every game, and I haven't found any genuine resources to properly analyse and explain why from a neutral perspective.

The short answer as to why Gas is overpowered is that it trivialises so many aspects of MvM and really throws Pyro ahead of the other classes, especially in the early game, of which Pyro already has a strong early game without it. In my opinion Explode on Ignite should be removed, and should be replaced with a slowdown upgrade, this would give the Pyro some much needed team utility as opposed to the huge amount of extra robot damage it gives now, of which Pyro already has a huge amount.

I tried to keep my analysis neutral all through this guide, but it's probably a bit obvious that I honestly hate this thing in MvM. Actually, I don't hate the weapon, I hate the Explode on Ignite upgrade, it's just too powerful in my honest opinion.

I'm probably going to get some flame for this, but I will never recommend using Explode on Ignite. I recommend using the Thermal Thruster, Detonator, or even just use Gas without explode on ignite. Explode On Ignite just makes the whole game unfun for everyone, even during the times I have used it myself for testing purposes (boot camp only) I don't find it fun, it's just boring.

I hope this guide gave you a better understanding as to why the Gas Passer in it's current state is so powerful. Let me know what you thought, any criticism is appreciated.

Edit - 5th August 2023

While I still believe the Gas Passer is overpowered, I have come to the conclusion that it is nowhere near the most overpowered thing in MvM at the moment. However I have drawn some other conclusions as to why this weapon is so hated, which I have explained below:

Pyro in MvM heavily relies on the Health on Kill upgrade to stay alive during engagements, since the Gas Passer has no such upgrade using it while in danger and in instances where health on kill is necessary to survive can result in more deaths and sub-optimal performance, essentially resulting in Pyros unintentionally griefing their team.

My stance on using Gas is still unchanged, I still believe both the detonator and thermal are both more fun and more practical for all skill levels, Gas should only be used by players who understand when to not use it to best avoid unnecessary deaths while maximising it's potential.