Got any more info?
Pc specs
What kind of addons?
Etc.
Btw, when you uninstall gmod, it only deletes the EXE. im not sure if you know, but cause this is the case, you could have dun-goof'd with a script or having a clash (unintentionally obviously)
Anywhore, try deleting your whole garrysmod folder and start from scratch.
If that doesn't work. I may consider sending you my log tool