As I said earlier
Make sure you are subscribed to all of this http://steamcommunity.com/sharedfiles/filedetails/?id=673094991
Then launch garrys mod, download the content, exit garrys mod, start it again, and attempt to join the server.
I have Episode one and two installed and mounted to garrys mod as well; not sure if that is needed though.
Other than that, if this does not work, then I do not know how to fix your problem friend.