At the login screen go to Sessions and select KDE. You should then be asked if you want to make it your default.
Alternatively on the settings menu (or similar) there should be an option for Login Window where the default session can be set.
Sorry I can't give you more precise instructions as I am on my laptop which still runs F8. My Desktop (with F9) is out of action.
when GDM Loads.. Click on your UserName an then you'll see underneath where it says Languages on the right hand side you'll see a GNOME an a Drop down Menu..you should be able to choose KDE from that drop down menu, then type your password in an Choose to make that as default DE