What can you say about a mod that has about a million servers and infinite replay value with it's own workshop and fanbase?
Well, for starters. this game has everything to offer due to its unbeleveable amount of mods and mod authors, you want to blow things up? Go download a c4 charge and plant it in a fully destructable building if your computer can handle it.
Want to watch youtube videos in a theatre? Go to a Gmod theatre server.
Want to live a life in a game? Go to a RP server.
Want to roam around a nuclear wasteland filled with zombies and bandits? Go to a dayz server or another end-of-the-world server.
Want to play sherlock holmes and find the traitor among the innocent? Or be said traitor? Find a TTT server.
My point is, you want something? there's probably a mod for that. If you have a friend that can play with you on a LAN server or just a private server, get creative in sandbox. you could make something, you could create scenarios, you could pose some ragdolls for some funny or gruesome screenshots, or even just send armies of rebels against two antlion guards. The choice is yours!
Overall, this game gets an 8 out of ten from me
The mod is the limit in this game, but unless you have friends. The servers can be hostile. (especially the TTT and DarkRP servers, you can get pretty paranoid in TTT )
And singleplayer is only so much fun, you really need to be creative to find fun in singleplayer.
I'm a bit on the un-creative side so give the game a try yourself, just make sure to bring a friend.