Gmod Startup Problem

Hi,two of my friends just told me that they are having problems starting up gmod, what is happening is that they click play and it lauching game but the game doesnt even open someone have a sugestion to this problem?

Make sure graphics drivers are up to date and try verifying integrity of game cache

Thanks. You were right, his graphic driver wasnt that good! He bought one yesterday, and now he´s playing again.:dance::quagmire: