I can confirm that the contents of /etc/gdm/custom.conf are IGNORED and the contents of /etc/gdm/gdm.conf-custom over-ride the settings in /etc/gdm/gdm.schemas, contrary to the documentation.
I can confirm that the contents of /etc/gdm/ custom. conf are IGNORED and the contents of /etc/gdm/ gdm.conf- custom over-ride the settings in /etc/gdm/ gdm.schemas, contrary to the documentation.