GDM doesn’t start up

If you are using systemd, check journalctl. If you see messages of the nature:

gdm[4607]: Child process -4635 was already dead.
gdm[4607]: Child process 4619 was already dead.
gdm[4607]: Unable to kill session worker process

Then you are likely suffering from bug #669146. For some reason, folders created by GDM at runtime do not have the correct permissions. To fix the issue:

# Replace .local/share with the problematic directory
chown -R gdm:gdm /var/lib/gdm/.local/share

You may also need to disable Wayland. Edit /etc/gdm/custom.conf: