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.