GNUStep is a cool idea (I’d love love love to be able to develop cross-platform desktop apps with Cocoa), but its problem is that it’s lagging so far behind that it’s practically unusable. If you’re targeting Apple Cocoa/OS X too (why wouldn’t you be) you’re going to be restricted to AppKit as it was back in OS X 10.5 or 10.6, and that sucks. Unfortunately, unless the project has a sudden windfall of cash and/or developers, I doubt this will change any time soon.