How To Select Your Log-in Manager in Ubuntu
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.