What I'm saying is, GNUStep is almost the same as the thing Mac OS X is built with!
I stopped reading right here. It doesn't sound like the parent knows very much about what he is talking about. Mac OS X certainly isn't "built with Cocoa." You may consider Cocoa to be a NeXTStep implementation integrated into the system. Furthermore, Cocoa and GNUStep are merely application development frameworks and have less to do with the workings of the system.