This is not a Guix bug. Closing. Very many GDM restarts is what happens on Debian Stretch and Arch (GNOME 3.32) as well after renaming /usr/share/xorg/modules, just like it does on Guix. On Guix’ wip-gnome3.30 GDM fails in a different way; maybe I will investigate further and then file a bug at GDM.
Regards, Florian
