You have to logout and login again to switch the desktop environment.
Click on the "sessions" button to choose the desktop environment you want before logging back in.
Just in case:
installation tutorial of gnome for Kubuntu:
http://www.psychocats.net/ubuntu/gnome
It's almost exactly the same for Debian if I remember correctly.
To get a pure Gnome desktop, you will have to remove the KDE desktop.
(logically by not using it while removing it I suppose, so either from a gnome session or a virtual terminal)
