Also (If you're paranoid to use a SVN and have something download+extract things for you - my dad always was), you can download the files themselves.
Sickness Models (Cars, etc.):
http://64.31.14.194/server/ocrp/Sickness%20Models%20Content.zipUI Fix (Black/Purple Checkers, etc.):
http://www.mediafire.com/?g1g1alriuah3c3gDownload the sickness models and extract the contents (Models, materials, all that stuff) to
C:\Program Files (x86)\Steam\steamapps\
YOURNAMEHERE\garrysmod\garrysmod
Or, if you aren't running the fancy win7 system and such:
C:\Program Files\Steam\steamapps\
YOURNAMEHERE\garrysmod\garrysmod
SVN is much easier (Which is why Crap-Head posted the link), but just in case - that's how you do it! ^_^