If you're using Cocoatron for your UI, your development time is as long as it takes for you to reimplement the necessary bits of UIKit, plus however long it takes you to make the rest of your application.
The same goes for GNUStep / the GNU ObjC runtime. Both of these projects have been focused on the Mac environment, not iPhone; they have AppKit, not UIKit.
Of course, you could write the core part of your program in ObjC and build against GNUStep/Cocoatron and use that elsewhere. But, at that point, why not just use C/C++ instead and libc/libstdc++ instead? It would be easier and save you a lot of headaches.
The same goes for GNUStep / the GNU ObjC runtime. Both of these projects have been focused on the Mac environment, not iPhone; they have AppKit, not UIKit.
Of course, you could write the core part of your program in ObjC and build against GNUStep/Cocoatron and use that elsewhere. But, at that point, why not just use C/C++ instead and libc/libstdc++ instead? It would be easier and save you a lot of headaches.