In case anyone else finds this through an internet search,
I've figure out how to make the fix work;
gdm3 uses the 'monitors.xml' format that gnome3 uses,
so if you are set to a different desktop manager (cinnamon, in my case) the
fix won't work.
Solution:
In the gdm login, click the gear and change to 'GNOME'
In gnome, click upper right corner and enter System Settings > Display
Configure Display as you like (you must make some sort of change to
recreate monitors.xml)
`sudo cp ~/.config/monitors.xml /var/lib/gdm3/.config/monitors.xml`
Reboot/relog and gdm should be on the correct monitor,
and you can switch back to the desktop manager of your choice.
