If you are missing CS:S Content, and all you see is purple checkerboards, follow this tutorial to fix the issue!
Download the content pack below and install it to the correct directory (listed below).
WinRar: http://www.rarlab.com/download.htm (Select the correct version for your OS)
CS:S Content: http://www.mediafire.com/download/xbnaj8e3ggds63y/CSS_Content_2014.rar
After you have downloaded that content, extract both of the folders to this EXACT path on your PC: C:/Program Files(x86)/steam/SteamApps/common/GarrysMod/garrysmod. Reboot the game and rejoin the server and you should be good to go!