This is very interesting. I believe I heard someone else say they had this issue before, but have since fixed it somehow.
Google says to try and uninstall gmod manually by renaming or deleting its folder completely (not from steams method alone), then reinstall it.
Or try deleting you cache folders: steamapps\common\GarrysMod\garrysmod\cache\lua
Google says to try and uninstall gmod manually by renaming or deleting its folder completely (not from steams method alone), then reinstall it.
Or try deleting you cache folders: steamapps\common\GarrysMod\garrysmod\cache\lua