When I was trying to install Gnome 3 on Ubuntu 16.04, just to get an experience, I got the below message. That dilemma forced me to search the site and found this and many others.Which display manager should I choose here? Is there a problem if I choose lightdm or sddm when installing Gnome?
The display manager is the program which displays the graphical login page. In practice there is very little to chose, they all let you chose a username and enter a password, and possibly choose what kind of graphical session to run.
lightdm is the default display manager which comes with Ubuntu.
gdm3, the Gnome Display Manager, is the default display manager for the Gnome desktop.