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Is there any way the framework could be linked up (efficiently) to KDE or Gnome (or even just QT or GTK+)?
I'm not sure I understand why you would want to do that. KDE and GNOME essentially offer GUI widgets, as does GNUstep. If you take a closer look, you will see that GNUstep's (or OPENSTEP, if you will) will offer a FAR superior concept in that respect.
Apart from that, to use Qt, you will need an Objective-C++ capable compiler, which gcc (currently) is not - a good thing IMHO if it stays that way.
But, there are 'bridges' for KDE/Qt and GTK to be used with Objective-C available. Take a look are Sourceforge or Freshmeet.