After installing gnoms-shell 3.26.1-0ubuntu5, I verified that I was able to install lightdm then uninstall gdm3 and I was still able to reboot adn log in to the default Ubuntu session. I was unable to uninstall gir1.2-gdm-1.0 without uninstalling gnome-shell.
After installing gnoms-shell 3.26.1-0ubuntu5, I verified that I was able to install lightdm then uninstall gdm3 and I was still able to reboot adn log in to the default Ubuntu session. I was unable to uninstall gir1.2-gdm-1.0 without uninstalling gnome-shell.