Gmod is my 2nd most played game on steam with 1,700 hours over several years.
It's a one of a kind game that revolves mostly around player mods. The most interesting of which are the game modes. Sandbox, TTT/Murder, Prophunt, and RP are the most notable ones, but there are some other good ones such as Zombie Survival, Deathrun, Melonbomber (Bomberman), and Stranded.
If you're going to be playing gmod for awhile, you'll definitely want to check popular servers out for different game modes. There's just so much unique content packed into one game. Now gmod isn't quite as big as it once was, and the game is pretty outdated in its tech, but that doesn't mean there aren't plenty of unique servers to play on still. You'd be surprised what's on here. 1942 rp and star wars rp both surprised me, with rather larger player counts. And even among tried and true game modes, like TTT, you'll find various servers with different rules and mods that keep the gamemodes fresh. One TTT might have some good maps and unique weapons, while others could have better T and D weapons and fun special rounds.
And while other games might have some of these game modes (TTT is common in other games, for instance), none are nearly as fleshed out as Gmod. Only gmod has access to so much.
This all being said, if you only want to play on single player or with a few friends on your own tiny server... you likely won't get too much out of it. Sandbox itself is kind of fun, but personally it gets boring after a couple of hours. Go into this game planning to try new things out and play on a bunch of servers, otherwise you might not find many hours to sink into it. Once you find a couple you love though... there's no escape.

Gmod is my 2nd most played game on steam with 1,700 hours over several years. It's a one of a kind game that revolves mostly around player mods. The most interesting of which are the game modes. Sandbox, TTT/Murder, Prophunt, and RP are the most notable ones, but there are some other good ones such as Zombie Survival, Deathrun, Melonbomber (Bomberman), and Stranded. If you're going to be playing gmod for awhile, you'll definitely want to check popular servers out for different game modes. There's just so much unique content packed into one game. Now gmod isn't quite as big as it once was, and the game is pretty outdated in its tech, but that doesn't mean there aren't plenty of unique servers to play on still. You'd be surprised what's on here. 1942 rp and star wars rp both surprised me, with rather larger player counts. And even among tried and true game modes, like TTT, you'll find various servers with different rules and mods that keep the gamemodes fresh. One TTT might have some good maps and unique weapons, while others could have better T and D weapons and fun special rounds. And while other games might have some of these game modes (TTT is common in other games, for instance), none are nearly as fleshed out as Gmod. Only gmod has access to so much. This all being said, if you only want to play on single player or with a few friends on your own tiny server... you likely won't get too much out of it. Sandbox itself is kind of fun, but personally it gets boring after a couple of hours. Go into this game planning to try new things out and play on a bunch of servers, otherwise you might not find many hours to sink into it. Once you find a couple you love though... there's no escape.