It's been addressed here GDM3 does not start in Ubuntu 18.04 but none of the solutions proposed worked for me (and the topic is about 18.04).
I have disabled wayland as proposed, but did not solve the problem.
If i connect thru SSH and
sudo systemctl start gdm it will work and start gdm. But if i boot the system, it just gives a blank (black) screen.
sudo systemctl enable gdm does not work. It gives a comprehensive message that it's not the correct way to enable this service.
How can I re-enable gdm?
Have also tried to uninstall and then install it again (the gdm package) - did not work.