Ubuntu 16.04 comes with LightDM as default display manager. But if you experiment with various display managers or various desktop environments, you may end up with GDM or some other display manager as the default display manager. It is a matter of choice but if you want to get back to another display managers, you can easily do that.
In this quick post, we shall see how to switch between LightDM and GDM or
Switch between LightDM and GDM in Ubuntu
Open a terminal and use the following command:
sudo dpkg-reconfigure gdm3
You can in fact use lightdm instead of gdm, the procedure is same for both. Using the above mentioned command will take you to a warning page. Enter to get back to next screen:
On the next screen, you’ll see all the available display managers. Use tab to select your preferred one and then press enter, Once you have selected it, press tab to go to ok and press enter again.
Restart the system and you will find your selected display manager at login. The same procedure can be applied to switch between GDM, LightGDM, MDM, KDM etc.