- In Steam right click gmod, properties, local files, Verify Game Cache (or something similar)
-If that doesn’t fix gmod then go to (unless you changed it) C:/Program Files (x86)/Steam/Steamapps/common/Garry’s Mod/ and delete everything in that.
-Then on Steam right click GMod and delete local content, then install gmod.
-If that doesn’t work make sure your graphics card’s drivers are up to date.
-Your pretty much screwed if that doesn’t work…
This is assuming you are on Windows btw.