You can use the old login screen if you want. Just remove kdm and install
gdm. Either will get you into Kubuntu or GNOME. The dropdown menu for kdm is
different. Click Session type at the bottom and it should reveal installed
desktops. Just click on GNOME and login. However, if GNOME is not in the
list then you have a problem. It means that GNOME is either not installed or
is missing parts. Note: KDE is different from Kubuntu and GNOME is different
from Ubuntu. When you install KDE and not Kubuntu you lose the default
Kubuntu packages and settings and get what KDE wants you to have. The same
for GNOME and Ubuntu.