Gmod can't start up.

I just got a new Asus G72gx with win7 64-bit, but when I run gmod, (clean install), the resolution goes REALLY small and then gmod just quits. There is no "hl2.exe has stopped working or anything. It just quits.

2.53Ghz Core 2 Duo
6GB ram
1GB DDR3 VRAM. Nvidia GeForce GTX 260m

I don’t see what is causing this…