Left 4 Dead 2 (If out on the PC)
or
Team Fortress 2?
I like L4D, but I also like Garry's Mod.
Sooooooooooo?

I'd get tf2, since it's cheaper. They are two different styles though so it depends on which you like.
If you do decide of l4d2, you should also consider the steam complete pack, since that's just 30-40 more. So price of one more game=everything. Great deal. I'd get it myself if I didn't already have tf2 and l4d2....and gmod. And css. I don't think they let you regift the newer games.
If they do, I'd like to know

Hmm, okay.
I'll consider