Non-steam games in GMod?

One of my friends just bought me CS:S, but he bought it (unfortunately) from Gamestop. I know that you can activate valve games on steam, but would they be mountable in gmod if I did that? Or am I going to have to buy a game that I already have to mount it?

Kinda need to know before I open the box, so I can return it if I wont be able to mount it.

Yes, because you HAVE to register Valve games on Steam. And Steam games by Valve are Gmod portable. As long as it’s a real copy (non-bootleg) and has NEVER been opened, it will work perfectly fine.

And trust me. I got retail copies of Portal, The Orange Box (I got Portal before), CSS, DODS, and HL2DM.

As long as it hasn’t been used previously, yes.

Alright, cool, thanks guys!