NOTE: a lot of people are still looking for css, ivecome across a lot of people that donthave it so please get these files from me. Its annoying to see people without cssand saying everything are errors. Imjust trying to be a nice guy.
Instructions:
1. download the files (obviously)
2. Extract the zip files to anywhere you can drag them later, i would suggest desktop.
3. Go to file explorer
4. Go to local disk> program files (86x) > steamapps> common > garrysmod > addons
5. take your file(s) and drop them in and start gmodor restart it if you were running it.
This post is really useful, although if you download the CSS textures and still have missing textures in servers that you play on, then: Make sure you have all the correct add-ons downloaded and installed for the server, and if they're still missing, download a few more content packs from here: http://kajar9.wixsite.com/cscheater2/downloads
If you're missing textures in Gmod, you won't need to download the CSS maps, or at least from my experience. They're just for you to download if you want to play Prop Hunt on de_office or something similar, even then it's likely that servers have some kind of modded map that allows you to download it from the Steam workshop.
"It never hurts to keep looking for sunshine" - A. A. Milne