First, go to

C:\Program Files\Steam\steamapps\<Steam Username>\garrysmod\garrysmod

Next, delete the Lua Folder (It will regenerate when gmod starts again, don't worry)

Now, Delete the Cache Folder (It will regenerate as well)

NOW, Delete the CFG Folder (it will Regenerate...)

and Finally, delete the sv.db and cl.db in the root garrysmod folder (C:\Program

Files\Steam\steamapps\<Steam Username>\garrysmod\garrysmod)


I would also run a virus scan on your "Steamapps folder"
C:\Program Files\Steam\steamapps\