To prevent misinformation ...
Many people have confused GNUstep with WindowMaker. GNUstep, however, is not a window manager. WindowMaker is the most often-used NeXT-looking application on a non-NeXT system. WindowMaker also uses a derivation of the GNUstep logo. WindowMaker is the preferred GNUstep window manager, but GNUstep applications also work with any window manager, although you're most likely, currently, to have a more cohesive desktop experience if you use the two in conjunction.
GNUstep is an advanced, cross-platform, object oriented environment composed of frameworks, tools and servers (daemons). It is very close to the Cocoa frameworks from Apple and tries to maintain compatibility with Cocoa wherever it is desired and possible. The roots of the GNUstep application interface are same as the roots of Cocoa: NeXT and OpenStep.
WindowMaker is a window manager, not a workspace manager nor a file browser. It is nothing more. WindowMaker and GNUstep share almost no libraries or functionality. WindowMaker is written in C, and GNUstep is written in Objective-C. WindowMaker does make certain things easier for GNUstep, but it is not GNUstep itself, although it is a part of the project.
In many people's minds the environment and the frameworks used to create programs merges. WindowMaker can be used with Qt or GNOME apps, just as easily as it can be used with GNUstep. Similarly GNUstep applications can run using KDE (Qt's preferred Window Manager) or Enlightenment (GNOME's preferred Window Manager). This misconception can make one loose sight of the powerful programming frameworks underneath.
WindowMaker is an excellent window manager and none of the preceeding should be construed to imply otherwise.