Should I buy Garry's Mod?

Yes and yes, it's not just a game. In Garry's Mod you literally have all of the freedom to do basically a lot. Explore new opportunities, create your own server, make workshop add-ons, LUA Coding, all that good stuff.

Bring your imagination in to the game, unlike Minecraft where you use blocks and stuff, you use props and learn all the tools provided by Garry's Mod such Welding Tool, Constraints, etc.

So yeah, plus it's a good game when you're bored. You can literally turn Garry's Mod in to another Game if you have the skills of coding in Garry's Mod.