You are most likely using LightDM (lightdm
package) as your login manager (as @Daniel T said) instead of the default GDM (gdm3
package). I suppose that you ended up with LightDM either because you didn't do a clean install, rather you upgraded from another installation where you had lightdm
installed, or because you followed some guide to configure your system and you installed lightdm
without noticing.
To remove lightdm
and replace it with gdm3
run the following commands in a terminal:
sudo apt install gdm3
sudo dpkg-reconfigure gdm3
systemctl stop lightdm
sudo apt remove lightdm
systemctl start gdm3