So first things first have you tried reinstalling gmod recently?
if not do it ans see the outcome.
Second of all this seems to me to be something to do with a dll file as its replacing one texture for another. so this is why i think it would be part of this but if not then you may need to uninstall css P2 hl1 & 2 or just delete all textures in the gmod folder and then go to steam and verify integrity of gmod. it sometimes fixes things like this GL speak to me in game or ts if you need more help