Hi there,
Please try closing your game, deleting your existing copy of the map from your PC, then opening GMod and trying to connect once more.
You can find your existing copy of the map in this directory, in Program Files (x86):
Just delete all of the files that have "rp_site65_civilgamers" in them. Once you've done this, the error should be fixed - this happens when your map becomes out of sync with the servers, so you need to delete it in order to download a fresh copy. If this doesn't work please post again and we'll try more troubleshooting. Thanks!