Help with GMod error SteamApp() is NULL

I just GMod yesterday and keep getting the error SteamApp() is NULL. It is a legally bought copy of GMod and I also have sources too. I have the CS source, Half Life 2 and Portal and all of these work without a problem. How can I fix this

It’d be helpful if your linked Steam profile was actually valid, for starters.

You could try verifying the game cache files though.

Whoops. Wrong thing. I fixed the link and I did try verifying the cache. Still not working