Ever Scince i put the option open in a window garrys mod wont open. I uninstalled the entire thing and its still not working. It will open for a split second in a window and then close all of a sudden? Anybody know how to fix the problem?
Well, what’s your desktop resolution?
Put it higher, to something greater than your gmod resolution 
Right click Gmod, click Properties, Set Launch Options, type “-fullscreen”