Edit: To be clear, GNUstep is something like 6 years _older_ than Mac OS X, so it's had plenty of time to catch up. The problem is really that these APIs are constantly changing and being extended. The goalpost is moving far too fast to ever meet it.
- Project Astoria[0], for porting Android apps
- Project Centennial[1], for porting Win32 apps
- Windows Bridge for iOS[2], for porting iOS apps
Project Astoria was later rebooted into WSL, and now with Windows Subsystem for Android we are back at Astoria's original goal.
[0] - https://en.wikipedia.org/wiki/Windows_10_Mobile#Project_Asto...
[1] - https://blogs.windows.com/windowsdeveloper/tag/project-cente...
[2] - https://blogs.windows.com/windowsdeveloper/2015/08/06/open-s...
And of course it wasn't so much "giving up" (which sorta implies it was the difficulty of the task), but more "losing interest", because they discontinued Windows Phone, where they were behind Apple's iOS in terms of market share. In that context, getting iOS apps on their system is economically interesting.
On desktop, Windows is way ahead of macOS in terms of market share, so there isn't much of a point.