There is a cracked version of Steam out there somewhere, and it tricks Steam into thinking you own every single game on the store, although you can't actually download the majority of them because of some server error.
A lot of people sell Garry's Mod at a really low price. You can have a look here if you want: Selling Accounts/Keys/Items - MPGH - MultiPlayer Game Hacking & Cheats - Hacks, Cheats, Downloads, Trainers, Games