What is there to do on gmod other than gamemodes??

Gmod is a sandbox game, so what ever you feel like doing.
Personally I enjoy going on the workshop and downloading a random map from the workshop and loading it up and seeing what it has to offer, you would be surprised on how many community made maps there are on the workshop.
Another thing I do is just download a bunch of npc addons from the workshop and load up one of my fav maps and make a big npc war and see who is left standing at the end of it all.

There is plenty of community made models, maps, weapons, tools and much more on the workshop to play around with you just gotta figure out how to put it all together and enjoy it!