Comment 116 for bug 1705369

I had same problem when disabled integrated card in bios. Found issue report in gdm3 https://gitlab.gnome.org/GNOME/gdm/issues/332. Issue description has link back to launchpad https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1716857.

Workaround mentioned in both reports - to change line in gdm.service to ExecStartPre=/usr/bin/xinit /usr/share/gdm/generate-config works for me.