- What Cocoa features does the application use? Something written for OS X 10.2 using "oldschool" APIs is much easier to port than a new app that perhaps makes extensive use of 10.5+ features, binding magic, Core Data, etc.
- What other frameworks does the app use? If it makes heavy use of something like Apple's media and animation APIs (Core Animation, Core Image, QTKit, etc.), it will be very difficult to port because GNUstep doesn't cover these.
(Edit: one important factor is the degree to which the port needs to integrate with the target platform. GNUstep doesn't look very native on Windows, for example. But if the port is something like a custom application targeting Linux, then the look of widgets and fonts may not matter at all.)
- What Cocoa features does the application use? Something written for OS X 10.2 using "oldschool" APIs is much easier to port than a new app that perhaps makes extensive use of 10.5+ features, binding magic, Core Data, etc.
- What other frameworks does the app use? If it makes heavy use of something like Apple's media and animation APIs (Core Animation, Core Image, QTKit, etc.), it will be very difficult to port because GNUstep doesn't cover these.
(Edit: one important factor is the degree to which the port needs to integrate with the target platform. GNUstep doesn't look very native on Windows, for example. But if the port is something like a custom application targeting Linux, then the look of widgets and fonts may not matter at all.)