Just maybe, this is a complete stab in the dark. Go to the properties of your GMOD (Right click in steam library,) go to Betas and opt into the Dev Branch. This may or may not work I have no idea. It's the last thing I can think of you haven't mentioned.

Just maybe, this is a complete stab in the dark. Go to the properties of your GMOD (Right click in steam library,) go to Betas and opt into the Dev Branch. This may or may not work I have no idea. It's the last thing I can think of you haven't mentioned.