You have both LightDM and GDM (in case you installed gnome-shell  this GDM display managers installed on your computer ). As in Ubuntu LightDM is default but you want to switch to GDM - to do this, run the command below in a terminal:

sudo dpkg-reconfigure gdm
Display something like this :

Select the display manager you want to use by default and hit enter. Then, restart your computer.