where is the KDE?
 

How can I use KDE in Debian etch? it seems it has installed by default only GNOME?

Is it install on your system? If so, you should be able to switch to it before you login to your system.

when I login there is only GNOME only. No KDE option

then install KDE
apt-get install kde

There is a Session type or whatever Gnome calls it that you can choose to use KDE, if you want then you can install the kdm package to use KDE's Display Manager when asked during the setup which to use as default choose kdm. Once done restart the X server then you need to choose again the KDE from the session type then it will be used as the default when logging in after this has been done until you choose another desktop to log into then it becomes default.