> GNUstep ran early MacOS apps on Linux after Apple bought Next and adopted their APIs.

No, it did not. There was no point at which GNUstep provided binary compatibility with Mac OS applications - while the two ecosystems shared some common history, it was much too far back to provide any form of compatibility. Additionally, Mac OS X used a different executable format than Linux or other UNIX-like operating systems, and GNUstep never attempted to provide that sort of low-level compatibility.

(For perspective: the GNUstep project started in the early 1990s. Mac OS X was first released to users in 2001.)

It may have been possible for some developers to port applications between Mac OS X and GNUstep while sharing some code, but that's a long way from running an unmodified application.