GNUstep Is Not a Window Manager
There is often some confusion about what, exactly, GNUstep is. GNUstep is a development environment. It’s a set of libraries and developer tools that can be used to write applications.
The most common window manager used with GNUstep environments is WindowMaker, although it doesn’t actually use GNUstep. Instead, it uses a library called WINGS (WINGS Is Not GNUstep) to implement some of the functionality of GNUstep in C.
GNUstep also is not "WINE for OS X." GNUstep implements much of the functionality of OS X, but at an API level. The ABIs are quite different, due to the fact that Apple maintains its own Objective-C runtime rather than using the GNU version.