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it appears that GNUstep is a rewrite of the Next/Apple display PDF system
Hm ... NO. First, NeXT didn't used a PDF display but a PostScript one. Secondly, GNUstep is an implementation of the OpenStep programming framework. Cocoa, on Mac OS X, is also an implementation (with some minor additions) of OpenStep.
The framework is where you find all the objects you need to program an application; a display PDF is just that, something to display PDF. Same for a display PostScript.
One of the interesst with the architecture of GNUstep (or Cocoa) is that you only write once your code for screen or for printing -- it's the same ! you send postscript to your screen and to your printer. The other big interesst is that theorically, between your screen and your printer, it's true wysiwyg.
Actually, there is no "useable" Display PostScript on linux, thus GNUstep provide a small additional level to print on non-postscript display (eg, X11 screen for example) ... for example the backend -art use libart to display GNUstep application (so it's smoothly antialiased). But you still send postscript code...
Anyway, the big interesst of GNUstep is that it is an implementation of OpenStep, which is one of the best programming framework available. And it's free.