You can use GNUstep Renaissance on top of the following OpenStep-inspired libraries:

• GNUstep, which runs on GNU(Linux) and most unix systems, and (experimentally) on Microsoft Windows.
• Apple Cocoa, which runs on Apple OSX.

Applications written using the OpenStep API for their core code, Renaissance for their user interfaces, and gnustep-make for their makefiles will build and run with almost no changes in source code under GNUstep and Apple OSX.