It would be trivial if the libraries ("frameworks") were the same; their sources are not shared openly by Apple. GNUstep was however based on the same open spec

It would be trivial if the libraries ("frameworks") were the same; their sources are not shared openly by Apple. GNUstep was however based on the same open specification as earlier NeXTStep / OpenStep libraries, which was the same foundation for Apple's current libraries. And porting is a popular use case, so compatibility can clearly be beneficial.