Garry's Mod (GMOD) is a standalone sandbox (formerly a mod of Half-Life) created by Garry Newman and managed by the fine folks at Facepunch.
GMOD is generally what you make of it. The relatively light Source engine allows it to be playable on decade old computers with integrated graphics. The lua scripting allows many gamemodes to be easily developed and deployed. Personally, I spend most of my time playing Zombie Survival. Overall GMOD is very recommended. 9/10.
Typical ZS Action, shooting zombies behind a cade.
Typical GMOD creativity within ZS.
Playing zombie.
Playing zombie, attacking a cade.
Retro TeamPlay.
Due to the fact that GMOD uses an older (2013) branch of source, as well as the challenges of getting lua multithreaded, performance is usually abysmal and very CPU limited. To help mitigate this, GMOD can be rendered in multicore mode by adding the following to autoexec.cfg:
gmod_mcore_test 1
mat_queue_mode 2
cl_threaded_bone_setup 1
This results in modest increases in performance; don't expect your 10 fps multiplayer game to suddenly jump up to what your GPU is capable of rendering.
If you're joining any online server, chances are that there's quite a bit of content that will be downloaded. It is usually located in garrysmod/download. For Steam Workshop content, it is in garrysmod/downloads/server.
A long time ago, GMOD required Counter Strike: Source to work. Now that requirement has been waived, but many resources still require it. If you do not have it installed, the infamous purple and black checkerboard pattern and ERROR appears. You can either buy Counter Strike: Source or grab the game data from somewhere else and install it into your gmod directory.