I believe the ideal solution would be to adapt the default GDM themes to use the common aliases such as Sans, Serif and Monospace, instead of specifying individual fonts.
I would mark this as an upstream bug for the GDM package at GNOME (hint, hint, who wants to do this?) -- gdm theme should use dejavu fonts https://launchpad.net/bugs/70829 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
