no steam apps

every time i click the garrys mod icon it will say that it cant sync my games to garrys mod and if i click play anyway it will get to the part where it shows the guy standing with the phys gun on his back and says its loading down in the bottom corner and the it will close and a little box will come up saying NO STEAM APPS! and thats it. I’ve tried everything can someone help

do you own any Source Engine games? like Half Life 2?

Yes, he does.

Did you restart Steam?

ya i already tried that