Aside from perhaps using some GNUstep classes for the non-visual parts, you'd need to separate the application's logic from its OS integration and then use whatever library you want to target.
One way to go for it would be to make your app use Qt (which is also available on Linux) instead of iOS-specific functionality and then port that Qt app to Linux. But whatever you do, if your app is made for iOS (or any other specific OS), you'd need first to untangle iOS from it and then make a port.
But FWIW i think fsflover had new applications in mind instead of porting existing ones.
