See the new guide here: [Guide] Installing Gmod Additional Textures (CS: S, HL2)
For any people who need the extra game textures for Garry’s Mod I found a person who compiled them all as textures.
REMOVED - Click on the GarryContent tab for downloads
Go to the link above and download the game content of everything you don’t have using the MEGA download (MEGA should get you the fastest download), you don’t need the maps just the game content. Once you have downloaded them just extract them into your Garry’s Mod addons folder