Never programmed under gnome (gtk) or KDE(qt) but those are two different graphical libraries, or framework if you want.
What that mean is that both use different function (method) call to achieve the (more or less) same result. They both do the same job (do graphical stuff), but they are entirely different and all of the GUI part of a gtk application would probably have to be redone to "port" it to qt.
More so, gtk is a C library while Qt is a C++ object oriented library.
But anyway, you can run a gnome app under kde anyway. Sure, they wont have a unified look and feel, but it will work.
Well, this is a pretty simplified explanation, sorry if you are not THAT newbie anyway :-)
Hope this helps :-)