/etc/X11/gdm/gdm.conf? I looked at that for awhile, and for grins I changed:
GraphicalTheme=Default
to:
GraphicalTheme=Bluecurve
It's now complaining about icon-language.png instead of background.png.
Did you try yum remove gdm && yum install gdm ?
No, I hadn't retried that, so I just did now. No change. The problem is somewhere else, somewhere in a recent update. I now have another server doing the same thing.
Jack