Since garry ported to steam wipe and steamcmd people are experiencing bugs and errors, so if you for example join our server and you see nothing but black and purple error signs then start counter strike source, it'll download everything needed in order to fix these errors. It works for some whereas it doesn't for others, so if you should still have the problem after starting counter strike source, then verify your game cache. For those who don't know how to
-Right click on garrys mod in your steam game library
-Go to properties
-Click on "local files"
-Click on "verify integrity of game cache"
That's it, if it still doesn't work after doing all that, then I have honestly no idea, I didn't have to download anything, everything was working fine for me.