If you have both KDE and Gnome installed on your machine and you have KDM ( KDE Display Manager) as the default login manager and you want to change the login manager to GDM( Gnome Display manager), simply type the following after opening a terminal.
sudo dpkg-reconfigure gdm
This will change your default login manager to GDM at the next boot.
I have done that, but it still starts with KDE.
why dont you remove kdm ?