Go to http://www.shiftout.com/wiki/index.php/Garry's_Mod/9.0.4 for a method of installing GMod 9
AtomicSpark is wrong. You do NOT need CS:S. I know this because I'm running GMod 9 without it right now.
I had this problem and I found a tutorial to fix it. Thought I'd post a link to it here in case other people are having this problem.
It's running fine, in a stable manner. So far the only reason I'd have to upgrade is that you can't join the latest version of the server which everyone else has...