Quick tip.
Long story short, I decided to swap out gdm for a lighter display manager on an underpowered machine.
I found the lxdm is packaged, but when I ran the system-switch-displaymanager helper script it told me that:
“The graphical display manager lxdm is not supported yet.”
I had a poke around in the system-switch-displaymanager code and found that it is very simple, and does this:
rm -f /etc/systemd/system/display-manager.service
systemctl enable $DM.service
Where $DM is what you pass it (one of GDM, KDM, XDM, WDM or LIGHTDM, currently).
So I checked if lxdm had a systemd service script (it does) and did that myself:
rm -f /etc/systemd/system/display-manager.service
systemctl enable lxdm.service
and it worked.