The Standart Gnome installed for Karmic don t give the change Login screen windows like Gnomes .
How to Downgrade your Gnome 2.28 to Gnome 2.20
First you have to stop your actual Gnome with :
sudo /etc/init.d/gdm stop
This command wiill drop your GUi Linux and you still in Text mode
Install gnome 2.20 with
sudo apt-get install gdm-2.20
This action will remove the 2.28 gnome and install the 2.20
Now This command to save the new GDM configuration
sudo sed ’s|X11R6/||’ gdm.conf >/tmp/gdm.conf
sudo mv /tmp/gdm.conf .
Yiur are ready to start your Gnome 2.20 with command :