I don't see why it would be a problem to at least try and cooperate with the GNU folks if it has a chance at reducing the effort and cost of having a operating compiler.
And like the man said in the article Redhat still owns copyright for some stuff. Also it seems that the GNUStep folks are interested in in Obj-C support even if it is not going to be 100% the same as what Apple ends up shipping. After all their versions of the OpenStep API is not aligned with Apple's version (Cocoa) and it does not seem to bother them that much.
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