In this tutorial we will see how to change your default display manager to GDM instead of LightDM. In first method we will see how to install gdm with the Software Centre and in second menthod we will do the same task through CLI
Switch to GDM with the Software Centre
Step 3 Restart your computer, and you should now have the “old” GDM login screen; note that you can still select from any available session, including Ubuntu (3D), Ubuntu 2D, Gnome, Gnome Classic, etc.
Switch to GDM via the terminal
Step 1 Open a terminal with Ctrl+Alt+T if you are on the desktop and not in the recovery console.
Step 2 Type
sudo apt-get install gdm
and then your password when prompted.
Step 4 Reboot the computer (you can do this with
if in the recovery console).