Should I buy Garry's Mod?

It's better to wait for the promotion, preferably 75% or more (it is quite often for this game). And the game is really great, but you have to have an imagination (if you play in the sandbox mode).
It offers many game modes, as if there were several games in one, most of the players created by the community.
The only thing that limits the pleasure of playing (apart from the lack of knowledge about the tools in the company) are the Russians, who are the lion's share of players. When entering the Russian server, I often met children who were chirping through the microphone, and the game does NOT give the possibility to silence the players, apart from being mutated by the admin or votemute.

With the help of the game, you get a lot of funny videos on youtube, just look