It's a sandbox game where you can build just about anything.
There are tons of custom games you can play within the mod itself and just...oodles of extra content you can download and use for free.
Honestly I don't really know how to describe it.
Waooo. I've been wondering what Gmod is too. Is it a like an actual game or is it more like putting togethor a 3d model with existing pieces and then doing whatever you want with it?