What can you do in garrys mod

Garry's Mod (or Gmod) is what's called a sandbox game. It lets you essentially do anything you want and add anything you want in the game. "Contraptions" can be made using props (these are just objects) and the various tools available. Tools include Weld (attaches props to things), Thrusters (i think this is obvious :), Wheels and heaps of others which I will not name. The things you have to use in the game is determined by the other Source engine games that you have. A minimum of one Source engine game is required in order to play but the more games you have, the more things will be in Gmod.
On top of having game content in Gmod you can download thousands of other things from websites dedicated to Gmod. Just search for Garry's Mod in Google and you should find them. There is also an ingame feature which allows you to get user- made content.