Isn't GNUSTEP, like OpenStep, a platform independent standard? If yes, then it would work on both GNU as well as non-GNU platforms, such as the BSDs. Also, how important is the GNU userland here - is it either an important part of GNUSTEP, or necessary for GNUSTEP to even work/run? If not, then leaving out GNU out of Linux doesn't mean much, since GNUSTEP could run on it, w/o things like glibc, x11 and so on.
GNUstep is completely platform independent. The only thing it requires is a POSIX layer for some of the low level functions (which is available on Windows in the form of MinGW). It abstracts the display and the events layers. GNUstep has implementations for X11 and for Windows and can have implementations for any windowing system you prefer.
The reason for the GNU in GNUstep is largely historical. Originally, GNUstep was supposed to be *the* development environment and windowing system for HURD and for Linux, but it didn't work out that way since KDE and GNOME came to prominence. GNUstep, since it followed OpenStep at the time and now follows Cocoa, had to be implemented against an existing spec as opposed to the "blow up the world and start everything new" philosophy of the other two projects. Implementing against an existing spec is much harder because it requires more discipline. Additionally, GNUstep is written using ObjC (for obvious reasons) and, back then, it wasn't as popular as it is now and developers interest both in ObjC and in coding for GNUstep were rare.