TLDR: A solid game that is further improved when played with friends. Even alone, a good community can make the game extremely enjoyable.

As you can see from my hour count, I have played Garry's mod for a long period of time. About 500 hours of my playtime is idle hours so for all intents and purposes, I got a ton of hours. You might think "How can one person get so much out of one game?"
As many others have said, Garry's mod, or Gmod, is more than just one game. While most games rely on dev built gamemodes or modifications of existing ones, community developers within gmod develop their own unique gamemodes that can be completely different from the base game.
From something as simple as a building-sandbox, to immersive, serious roleplay; from base building attack vs. defense, to even RTS gamemodes, there is simply a stunning amount of diversity.

In terms of graphics, the game is certainly a hit or miss and greatly depends on the gamemode; same deal with sound design.

Now, in regard to performance, this is where things get dodgy. Gamemodes within gmod run off of LUA. Certain elements within LUA, in the hand of an inexperienced developer, can be extremely laggy. A system that would net consistent 300 FPS on CSGO might struggle in certain gamemodes in gmod for seemingly no apparent reason. The same deal is with some of the maps. Some maps in gmod are depressingly unoptimized to the point where even the slightest amount of optimization would improve framerates from a depressing 30 fps to the triple digits.

The community in gmod greatly depends on what community you as a player join. Every community is different. While one community might encourage being friendly to one another on top of strict rule enforcement, another community is the rough equivalent to a complete moral free-for-all. In general, there is a mix of both young and old (old being 20+ yrs) players.

Recommendations: For those who are interested in playing gmod, as a longtime player I have a few recommendations. First, consider investing in getting Counter-Strike: Source and Half Life 2. Having CS:S, HL2 (with all the episodes) will greatly reduce the amount of errors you will see as you play the game. While you will still occasionally see an error from missing content (from a missing addon), the odds of you missing content from one of the few games you don't own are extremely low. For perspective, in my entire time playing gmod, I saw errors from not having Day of Defeat Source only three times.
A microphone is highly recommended. Even if you are young, so long as you don't scream your head off, only someone with a depressingly shallow life would actually care.

TLDR: A solid game that is further improved when played with friends. Even alone, a good community can make the game extremely enjoyable. As you can see from my hour count, I have played Garry's mod for a long period of time. About 500 hours of my playtime is idle hours so for all intents and purposes, I got a ton of hours. You might think "How can one person get so much out of one game?" As many others have said, Garry's mod, or Gmod, is more than just one game. While most games rely on dev built gamemodes or modifications of existing ones, community developers within gmod develop their own unique gamemodes that can be completely different from the base game. From something as simple as a building-sandbox, to immersive, serious roleplay; from base building attack vs. defense, to even RTS gamemodes, there is simply a stunning amount of diversity. In terms of graphics, the game is certainly a hit or miss and greatly depends on the gamemode; same deal with sound design. Now, in regard to performance, this is where things get dodgy. Gamemodes within gmod run off of LUA. Certain elements within LUA, in the hand of an inexperienced developer, can be extremely laggy. A system that would net consistent 300 FPS on CSGO might struggle in certain gamemodes in gmod for seemingly no apparent reason. The same deal is with some of the maps. Some maps in gmod are depressingly unoptimized to the point where even the slightest amount of optimization would improve framerates from a depressing 30 fps to the triple digits. The community in gmod greatly depends on what community you as a player join. Every community is different. While one community might encourage being friendly to one another on top of strict rule enforcement, another community is the rough equivalent to a complete moral free-for-all. In general, there is a mix of both young and old (old being 20+ yrs) players. Recommendations: For those who are interested in playing gmod, as a longtime player I have a few recommendations. First, consider investing in getting Counter-Strike: Source and Half Life 2. Having CS:S, HL2 (with all the episodes) will greatly reduce the amount of errors you will see as you play the game. While you will still occasionally see an error from missing content (from a missing addon), the odds of you missing content from one of the few games you don't own are extremely low. For perspective, in my entire time playing gmod, I saw errors from not having Day of Defeat Source only three times. A microphone is highly recommended. Even if you are young, so long as you don't scream your head off, only someone with a depressingly shallow life would actually care.