Took me a while to follow. So this is step one (and step two might be to move lightdm.service from the lightdm bee package to mxtools and step three might be to remove ConditionPathExists=/node/tags/lightdm from it, right?). Looks good, go ahead.

The file had to be manually removed (of course in hindsight): sudo rm /etc/systemd/system/gdm.service.

Yes, mxtools has no "uninstall". And no auto-disable, too, so you left a dangling symlink. :-)

Indeed. Thanks.

@deinemuddah:~$ ls -l /etc/systemd/system/graphical.target.wants/gdm.service
lrwxrwxrwx 1 root root 31 Mar 14  2011 /etc/systemd/system/graphical.target.wants/gdm.service -> /etc/systemd/system/gdm.service
@deinemuddah:~$ sudo rm /etc/systemd/system/graphical.target.wants/gdm.service
@deinemuddah:~$