I receive this error every time I try to load up gmod:

I've done what it suggests and restarted steam but with no success. Does anyone know any other way to solve this?

I receive this error every time I try to load up gmod: https://gyazo.com/5f8b3973b12eb5a545b62dfca912880b I've done what it suggests and restarted steam but with no success. Does anyone know any other way to solve this?