The default display (login) manager for Ubuntu/GNOME is GDM. KDM, which is the login manager for Kubuntu/KDE is installed when you install the kubuntu-desktop package in your Ubuntu system.
If you want to switch back to GDM as the login manager, run the following command at the terminal;
sudo dpkg-reconfigure gdm
At the ncurses menu, choose gdm and click OK.
You'll now be presented with GDM every time you boot your Ubuntu system.