Cool! I went into debian for my first install from knoppix too. When u boot the computer it should take you into a login screen. There, on the bottom right corner is a menu that will let u choose what desktop you want to boot to.
If you want to use the kde login screen, open package manager and install the kdm package, then uninstall the gdm. If you wish to dump gnome entirely, you may go under the gnome section of your installed packages and uninstall the desktop and whatever extra gnome packages you don't want. Also, there are other gnome packages scattered throughout the package manager (lib, etc.) you will need to find any of these that are not necessary for kde and uninstall them.