Honestly this is everything you could want in a game. Not many other ways to say it. You make gmod what you want it to be. The possibilities are endless and it's hard to get bored of it. It's relatively cheap too, so there's really no downsides to it at all.
And I'm not even kidding about that, I've been playing Garrysmod for atleast 5 years (with some other accounts too.) The addons are basically what makes this game, THIS GAME! And the Multiplayer servers, so many delightful gamemodes to play! You can choose to be a total goofball in Prop Hunt, or choose to be serious in DarkRP, the choice is yours!
Afterall, it is a sandbox game. I totally recommend it, so many hours put to play this game. . .