I was looking for a way to switch to
lightdm, when I found this answer.
So I tried to apply it, and entered
sudo dpkg-reconfigure gdm3, then a selection screen appeared with 2 options:
lightdm. It was the same screen that appeared during the installation of GNOME (using
sudo apt install ubuntu-gnome-desktop && gnome-shell). I chose
gdm3 then pressed Enter, then I got this error:
gdm.service is not active, cannot reload. invoke-rc.d: initscript gdm3, action "reload" failed.
It turns out that it just required a reboot to work. But I still want to know the meaning of that error I got. Does anyone have any explanation?