Well, I'm sure reasonable people can disagree on this, but my opinions are:
* Cocotron and GNUStep are wonderful projects and the devs should be very proud of their accomplishments.
* But for real-world, shipping applications, the only cross-platform environment worth a damn is the Web.
* If you're not going to develop a webapp, then you should always develop apps with the native application toolkit for the target platform. To do otherwise leads to poor quality and major hurdles that are not worth the effort to overcome.
* So if I were going to develop a Windows app, I'd use the native Windows toolkit (whatever that is these days, I'd have to research).
* And besides, Schwartz takes advantage of several 10.8 Cocoa/AppKit APIs, and is of course completely entwined with my Xcode 5 project. I'd be surprised if Cocotron or GNUStep are that up-to-date. I suspect porting the project to Cocotron or GNUStep would be a major undertaking.
Glad to hear about "Onyx" though. Very cool that someone did an independent implementation of Quartz. If I were to do a Windows version, I'd probably combine Onyx with a native Windows app. But I don't have the time or expertise to make that feasible.
Hi, developer of Schwartz here. Yes, this is a thoroughly Mac-native application, so I won't be able to make it available on Windows.
Aside from the considerable difficulty of finding or developing an implementation of Quartz 2D that would run on windows, the app itself is mostly Cocoa/Objective-C (with a little Python of course). The entire app would have to be re-written from the ground up for Windows.
Word.