I think (and I'm going on memory here) that you should:
Boot the fedora dvd or cd into rescue mode, or whatever they call it.
If using the dvd, you should get instructions on how to chroot the sysimage. Follow these.
Go to /etc/pam.d ("cd /etc/pam.d), then "cat gdm"
Hopefully you will see a line similar to the one you commented out (auth required pam_succeed_if.so user != root quiet)
If not try "grep root *" and see what the filename is that has that line in it.
Edit the relevant file and restore it to what it was (hopefully just uncommenting, but I suspect you've done something else to it as well)
See how you go.