Try TF2, I think you need to play it at least once for GMod to work right.
Uh, you need a Source game for it, so probably. Get VALVe games, now. But not L4D if you're using it for GMod, I don't think it works, unless it gives you the SDK.
Once again:I bought TWO GAMES. Gmod and Half Life 2.
Then try running Half Life 2 first, then Gmod.
I tried to run HL2, but it got stuck on loading and won't finish. It's fried for some reason.Still no fix for Gmod.