GDM3 does not start in Ubuntu 18.04

I had a similar problem updating from 16.04 to 18.04.

The solution in my particular case was to deactivate Wayland modifying /etc/gdm3/custom.conf uncommenting or enabling WaylandEnable=false so the GNOME display manager will always load the GNOME desktop environment from gnome-desktop and not Wayland. This is because Wayland usually has errors with some graphics drivers.

This can be done easily from an Ubuntu live USB or if possible start in Recovery mode from the grub menu.

I was a victim of this issue for some time with Ubuntu 18.04 on a Dell Latitude 5580 which uses nvidia. I think gdm is not fully compatible with this spec.

After trying many solutions, including reinstalling Ubuntu several times, I decided to shift to LightDM and that fixed the issue.

In the booting screen press Alt+F2 and execute these commands:

sudo apt-get install lightdm
sudo dpkg-reconfigure lightdm

This worked for me. Good luck :)

I found a solution based on this and it worked for me.

First:

sudo apt install ubuntu-gnome-desktop

And then:

sudo apt install gnome-shell gnome

Next restart the system or simply sudo systemctl restart gdm3.