Group OP's guide is... well not as good as i expected. So, Valve broke alot of stuff with steampipe in 2013 making gmod9 unable to mount css and hl2dm by itself. The Group OP's guide will give you unstable game that crashes often.

You can get patched copy with all content game needs from "https://gmod9.com/download". After downloading 7z package, you have to extract it using WinRAR or 7zip. after extracting, copy gmod9 folder to Steam\steamapps\common\sourcemods\ and restart steam, if you have gmod9 installed just override files. Restart steam and you can play gmod9.

Playing in windowed mode is advised because fullscreen causes game to crash more often.
Edit: Steam flagged gmod9.com as potentially malicious. Copy and paste the link.