Cocotron
An alternative to GNUstep for cross-platform ObjC development, that seems to be generally much better, is Cocotron. I’ve looked at it a few times. Info can be found here:
A really nice feature that it claims (I haven’t tried it) is that you can compile from Xcode on your Mac and have it build a Windows binaries.
Unfortunately it is incomplete. One of the things it lacks is support for Key-Value coding, which is one of the basic elements that is required by the MaxObject framework I’ve been working on and the ObjC version of TTBlue.
That means I can’t use it.