Major GMOD error - over half a year of no fixes

[Terran]

Lets see, did you try
Making sure that Steam is up-to-date?
Making sure that steam is actually running?
Restarting your PC?
Deleting system 32?
Deleting the "ClientRegistry.blob" file and then restarting steam?
Deleted Garry's Mod local content?

If you solved it via that, please let us know! :)