GMod Sandbox is capable of a lot of things as a lot of us may already know but I know that people would love to create their own DayZ styled experience all by themselves without the hassle of downloading tons upon tons of addons to enter a server that will probably just kick you out before you even start (whether it be connection, missing maps, etc.).

But then again, creating this DayZ style experience will make you have to individually place one zombie down at a time right? Wrong. With just a couple of little addons you can make yourself a great DayZ-like experience.

First you will need the addon called 'Zombie/NPC Invasion': http://steamproxy.net/sharedfiles/filedetails/?id=179517028&requirelogin=true
This will allow random enemies to spawn on the map at any given time as long as you enable the addon in the options menu - with this addon it will allow you to customise what NPCs you want out on the map whether it be zombies, antlions, combine, you name it. Add the enemies you want and tinker with their statistics if you so desire, create boss enemies or make them spawn in large hordes to attack you.

Secondly you will need a fairly large map to play on, I suggest GM Arid Valley V1: http://steamproxy.net/sharedfiles/filedetails/?id=186587611&requirelogin=true This will allow all your enemies to roam around freely and do whatever they like around the map as well as give you the ability to explore a zombie-infested wasteland. The Headcrab remover is a good addon to have if you exclusively want non-headcrab zombies: http://steamproxy.net/sharedfiles/filedetails/?id=163658225&requirelogin=true

I also recommend a weapons remover addon also, in case you would want to start off with minimal weaponry and treat it as a survival game. I recommend this:
http://steamproxy.net/sharedfiles/filedetails/?id=151564834&searchtext=weapon+remover%2Fgiver
You may also find this addon useful:
http://steamproxy.net/sharedfiles/filedetails/?id=199226357&searchtext=loadout
I tested this and it works well with addon weapons but I can't seem to get it to work with Half Life 2 weapons for some inexplicable reason.

Item crates and other things for use for immersion or other purposes:
http://steamproxy.net/sharedfiles/filedetails/?id=112234141
http://steamproxy.net/sharedfiles/filedetails/?id=104619813&searchtext=Half+Life+2
http://steamproxy.net/sharedfiles/filedetails/?id=105565372&searchtext=Half+Life+2
http://steamproxy.net/sharedfiles/filedetails/?id=111335690&searchtext=Half+Life+2
http://steamproxy.net/sharedfiles/filedetails/?id=153246356&searchtext=Half+Life+2


Now if you're playing in single player then you are ready from the get-go but if you are going to be doing this in multiplayer then you can play this competitively or cooperatively - or both if you like to, however you like but it will require a touch of roleplay however meaning you will have to create your own goals. It is a sandbox game so feel free to do whatever you like, build a house, hunt zombies, scavenge combine bodies for ammo, have a clan battle with your friends with zombies out on the field. It is entirely up to you how you want to play the experience with your own rules and goals set in mind.

Now you have the nessessities nailed down you can start there if you like but you can customise the rest of your DayZ-like experience however you want, such as adding in new guns, new vehicles, objects, whatever you like - it's all up to you. Actually here are a few weapon addons you may like:
http://steamproxy.net/sharedfiles/filedetails/?id=197240079&searchtext=M9K
http://steamproxy.net/sharedfiles/filedetails/?id=128093075&searchtext=M9K
http://steamproxy.net/sharedfiles/filedetails/?id=128089118&searchtext=M9K
http://steamproxy.net/sharedfiles/filedetails/?id=128091208&searchtext=M9K
http://steamproxy.net/sharedfiles/filedetails/?id=144982052&searchtext=M9K
http://steamproxy.net/sharedfiles/filedetails/?id=173542267&searchtext=M9K
http://steamproxy.net/sharedfiles/filedetails/?id=173540899&searchtext=M9K
http://steamproxy.net/sharedfiles/filedetails/?id=172508645&searchtext=M9K
For people who have Counter Stike: Source mounted might want to try this:
http://steamproxy.net/sharedfiles/filedetails/?id=143193342
Now some handy M9K weapon commands if your computer lags when firing them (Due to the gas effect):
M9KGasEffect 0 - To turn off the really laggy and annoying gas effect
M9KDisablePenetration 1 - To disable bullets penetrating through walls, enemies and objects
M9KWeaponStrip 0 - To stop weapons from automatically dropping if you run out of ammo
All guns in the addons should use standard HL2 ammo so any HL2 weapons you pick up will contrubute as ammo for the addon weapons, I really hope these help.

Have fun, I hope you find this guide useful to create your zombie infested wonderland.

Credits:
Sonicdaheg for the Zombie/NPC Invasion addon
Frosted Koolcat for the Headcrab Remover addon
Salamander for the GM Arid Valley V1 addon
GigaXephones for the Weapon Remover/Giver addon
Joe Mantis for the Half Life 2 Setting addon
Rubat for the Half Life 2 Tools addon
Nekras - Andy Velandus for the Half Life 2 Tools Extra addon
ErrolLiamP for the Drop Weapon on Death addon
gorea235 for the Weapons Loadout addon
[RTKZ] Zombie Extinguisher (Be) for the S.T.A.L.K.E.R weapon pack
INCONCEIVABLE! for the four M9K weapon packs
Butch DeLoria for the Near Future weapon packs
Spy and tayley for the Customizable Weapons addon
Tape and Pyromaniac for the CrateMaker addon

Message or comment if you want your addon featured on this guide that will benefit the DayZ-like experience.