I had something close to this problem with my Arch linux, but I could log into the Gnome after a secound try on gdm. my /etc/inittab file has got a
line in the bottom which in installing gnome according to the wiki, it should be uncommented, and that's why I got the " there already apears to be an X server running"
I solved this by taking that line back to comments.
easy, effective and still have the gdm!