When Steve Jobs created the first mac, he tried to replicate the look and feel of the Xerox Alto. He did not pay much attention to other aspects such as Smalltalk-80 and object oriented programming.
NeXT is an attempt to incorporate those concepts that were initially left out from the mac, thus creating an experience that is more faithful to the original Xerox Alto. That's the origin of Objective-C.
It may be possible to create something like macOS based off GNUstep. There is a project going on that direction... Etoile. http://etoileos.com/etoile/
There's actually quite a difference between Classic Mac and Alto/Smalltalk. (Both sides would argue that the other one totally missed it. There's even some evidence hinting at the Lisa having experimental windows even before the PARC visit, and these screenshots look actually more like the final product than what came after the visit. [1]) Just install an Alto emulator and try it yourself. Also, the Mac System was famously programmed in Object Pascal (like the Lisa).
I really like the idea of Étoilé, but a lot of the docs are outdated because of the death of Gna!, and right now the focus is more on the CoreObject framework rather than the UI.
NeXT is an attempt to incorporate those concepts that were initially left out from the mac, thus creating an experience that is more faithful to the original Xerox Alto. That's the origin of Objective-C.
It may be possible to create something like macOS based off GNUstep. There is a project going on that direction... Etoile. http://etoileos.com/etoile/