GNUStep[0] is a thing that exists, and it's mostly Cocoa-compatible (at least for old versions of Cocoa)? I don't think it is very widely used -- it never got much desktop traction compared to KDE or Gnome. But you can write apps that compile and run on both GNUStep and macOS -- the "macOS" version of Emacs actually targets GNUStep, for example.

[0]: http://gnustep.org/