Sorry it's taken so long to get you an answer, but I'd not noticed your question before for some reason. I'd had the same issue, and as it turned out, gdm was looking for a missing database. Use ^Alt-F2 to get to a text console and log in as root. Then, run this command:
yum reinstall gdm
After you're done, reboot and it should work. Good luck and, again, sorry for the delay.