How To Install GDM On Ubuntu 14.10
The GNOME Display Manager (GDM) is a program that manages graphical display servers and handles graphical user logins.
GDM provides the equivalent of a “login:” prompt for X displays: it asks for a login and starts X sessions. It provides all the functionality of XDM, including XDMCP support for managing remote displays, and extends it with the ability to start X servers on demand. This package contains the next generation GDM, which was developed using the technologies on which GNOME 3 is based.
To install GDM open a termnal window (press ctrl+Alt+T) and copy+paste the following line:
sudo apt-get install gdm -y
During the installation you need to configure GDM and when asked select gdm from the list and press enter. When the installation is finished you can close the terminal window, reboot your system and enjoy the new login screen.
If you want to revert the changes open a terminal window and copy+paste the following line:
sudo dpkg-reconfigure gdm
Select lightdm and reboot your system.