Validate Game Cache, To do this go to Steam Libary > Garry's mod > Right Click > Properties > Updates > Validate Game Cache,
Make sure you have CSS ticked in the main menu of Garry's mod.
Also moved this to the correct section for you!

Validate Game Cache, To do this go to Steam Libary > Garry's mod > Right Click > Properties > Updates > Validate Game Cache, Make sure you have CSS ticked in the main menu of Garry's mod. Also moved this to the correct section for you!