Gershwin was the codename for an ambitious OperatingSystem that was supposed to be released as MacOS 9 by Apple in 1998. Successor to the Copland project that was cancelled in 1997, it never really got off the ground and rumor has it that no programmers were ever actively working on Gershwin. The version of MacOS widely known today as MacOS 9 bears little resemblance to Gershwin although it undoubtedly contains some features that were enhanced from their origins in the Copland project, most likely with ideas developed for Gershwin. Some people have tried to suggest that Gershwin was simply renamed to Rhapsody? and while Rhapsody was certainly closer to the ideas of Gershwin than MacOSX when it was released, they are fundamentally different OperatingSystems with Rhapsody being based on NeXTStep?.
The premiere of Mac OS X Server 1.0 was probably the closest OS ever released to match the original preliminary vision of Rhapsody. However, today the modern codebase of OS X server 10.2.x is on par with Mac OS X 10.2.x, plus additional handy server GUI configuration & maintenance facilities; So it no longer resembles Rhapsody/NextStep any more than the regular "client" version of Mac OS X and any NeXT-ish? apps need to be tweaked slightly to compile properly.