GMod refusing to start up.

Can anyone help me with the problem I am having with GMod? Whenever I try and start the game I get an error saying something about my local files needed to be downloaded from the server again, and I press OK on that. Then the game keeps on loading till that comes up again. And when I try and install it again, I get an error with the Steam servers. So at the moment I can’t play GMod to test a new model that I put together. Can anyone help me with this?

Go into your steamapps folder, rename the garrysmod folder to something else, then start the game all over again, see if it works. If it does, you might want to try and re-add stuff (addons etc) one at the time to see if an error occurs.

Thing is though, before my computer was playing up and I got XP reinstalled, it worked fine.

I found out that there is something wrong with my GMod .gcf. Is there anyway to get this not through Steam?