ubuntu 18.04 gdm3

change to lightdm

case 1

sudo apt update
sudo apt upgrade
sudo add-apt-repository main
sudo add-apt-repository universe
sudo add-apt-repository restricted
sudo add-apt-repository multiverse
sudo apt install lightdm

case 2

sudo apt-get install lightdm
sudo dpkg-reconfigure lightdm

config gdm

sudo apt install ubuntu-gnome-desktop

sudo gnome-shell gnome

sudo systemctl restart gdm3

purge gdm

sudo apt purge gdm

ref

https://askubuntu.com/questions/1050672/gdm3-does-not-start-in-ubuntu-18-04