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GNUStep doesn't run well on Windows right now. GNUStep isn't a viable development platform for Windows, YET. Hopefully, it will become much better since, beyond the crossplatform capabilities, it allows developers to create more complete and elegant applications more quickly.
There are a few compatibility barriers between Cocoa and GNUStep. The first would be the nib format used by Interface Builder. You can get around that by using 'nib2gmodel' which will convert nib files into GNUStep's gmodel files. You can always just use Gorm to re-create the interface for GNUStep if you don't want to convert it. GNUStep doesn't currently support Objective-C++. That isn't a big deal unless you are tring to write a wrapper around a currently existing back end (Obj-C++ is what allows applications like Safari or Camino to exist - those people took a C++ project and quasai-ported it to cocoa).
Beyond that, avoid using Cocoa.h includes, CoreFoundation, Quicktime, NSToolBar, or Drawers.
The two are VERY close because GNUStep was designed to be a "clone" of OPENSTEP which Apple didn't modify much.
Oh, it might be easier to write for GNUStep and "port" to OS X since OS X supports pretty much everything that GNUStep does so you don't run into the worry of "I designed this app around Quicktime or something else and Quicktime or something else isn't available on GNUStep." I listed out what to avoid, but sometimes one forgets and uses something and then you've got to take it out. Either way it's not bad.
I'm sure that some of the people working on the project would help you if you asked.