If you've added the Kubuntu packages to Ubuntu, or vice versa, then you can choose whether you want to use KDM or GDM to act as your log-in manager.
To choose your log-in manager, open a terminal and enter
It doesn't matter. Either one will work.
Hit [Enter] on your keyboard to move past the message screen.
Use your arrow keys to select gdm or kdm and hit [Enter].
That's it. Your chosen log-in manager will show up the next time you boot your computer.