Help with CS:S Textures not showing up in Garry's Mod

[ItzPixel]

My friend recently gifted me a copy of Counter Strike:Source. When I join any server that uses these textures it just shows ERROR! When I click on the gamepad on the bottom it shows an X for counter strike. Can someone please help.

[Captain Charles]

Launch Counter-Strike: Source and load a map, at least once. Then try mounting the game.