Garry’s Mod is for sale through Steam. If you don’t have Steam you can download it for free and create an account here. Steam is kind of like iTunes, for PC games.

You can buy GMod from the Steam store here.

Important: To buy Garry’s Mod you need to already own another Source Engine game on your steam account. If you don’t, you can order it with one of them in a package. I recommend the Counter-Strike and Garry’s Mod pack.