Originally Posted by
iamrubbish
do you need to have bought and installed half life 2 to be able to use gmod?
No, you can do it if you only own DoD:S or CS:S or something, although it's very recommended you do have HL2 for online games. Most online servers use Half-Life 2's assets, and if you run Gmod without them you're going to be missing out on a lot of props and stuff other people make.
To get Gmod to run at all, you need a Source game. I highly recommend having both HL2 and Counter Strike: Source purchased and installed, as you're going to see missing textures and models without owning both. Of course, the more games the better, but if you're starting from scratch I'd go with those two.
Also, even if you have HL2 installed, it won't work unless you actually
own the game on the account you own Gmod on. A torrent or gift pass won't work.