Mac developers have no need to contribute to GNUstep. We have a
better version already.
Better? Foundation is sometimes very (!) annoying because of its
CoreFoundation! Try e.g. to write a transparent persistence layer.
Modestly said: very, very ugly.
Or all those (void*) as instance variables. Impossible to handle. So
on the one hand, you have a beautiful language like ObjC with a lot of
runtime information and on the other hand you have Apple's Foundation
implementation.
Distributed Objects are also better in GNUstep.
And if you have a problem, you debug lightning faster in GNUstep,
because you trace through the source of "Foundation". Actually the
only things, that are better on MOSX are AppKit and Xcode.