It would be essentially impossible, they've essentially gotten the latest version of Obj.C. working (which is awesome!), but they don't have any of the frameworks from iOS/OS X reimplemented. The frameworks make the language, so you wouldn't be able to port any code at all, really.
I think you'd eventually need to compile with the apple toolchain to get the code signing but other than that if the APIs are there you should be able to do a lot of the dev with GNUstep.
The APIs are exactly what aren't there. That is, the runtime APIs are now there, but the framework APIs aren't.
GNUstep's runtime just gained some language/runtime APIs, that is very low level features that appeared in Apple's recent updates to their implementation of the Objective-C language & runtime. This is a very different matter from the gargantuan frameworks (Cocoa, AppKit, UIKit) which Apple has built atop Objective-C. These frameworks are the APIs that should be "there" to allow doing "a lot of the dev with GNUstep"— an they are a long way from being implemented by GNUstep, but some are indeed being worked on.
Off-topic but I thought it worth mentioning that Microsoft's Autheticode code signing (used to digitally sign executables) works just fine with the gcc/g++ tool chain. Maybe Apple's can too?
