It's basically the jack of all trades of games. Wanna play dodgeball? There's a gamemode for it. Want to play murder mystery? Sure, why not. Want to load up a random sandbox map and then nuke it so hard you crash your friend's computer? Yeah, you can do that.

It's got some age to it, but don't let that deter you from playing it. It holds up really well and the experiences you can get in gmod are easily unique. For better or for worse lol.