I bought Garry's Mod, and it's currently a 2 gb download. I have poor wifi, so I want to know if I can transfer Gmod from my friend's account to mine. He has Gmod on Steam as well. Is it possible with the backup and restore method and a pen drive?
You should be able to do the backup of the game and then restore it on to your machine via the Steam UI.
I have successfully done it by just copying the folder that contains the game and transferring that via USB drive. Probably quicker, but it will have to verify the files when you try and 'install' it on your computer, but that is usually a quick process if his game files are up to date.