It doesn't look like a gdm issue to me. The file
'/usr/share/images/spacefun-wallpaper.svg' is owned by package
'desktop-base', not gdm. I can login fine, just can't select
and use the spacefun-wallpaper background. The gdm
version is the same on both machines (2.20.11-4),
and gdm is not run on the VNC session in any event.