GNUstep is NOT Window Maker
In order to prevent misinformation ...
GNUstep is NOT a window manager
Why is this confusion floating around? Window Maker is the most often-used NeXT-looking application on a non-NeXT system. Window Maker also uses a derivation of the GNUstep logo. Window Maker is the official 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.
What is GNUstep?
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: Ne XT and OpenStep.
Why is GNUstep using WindowMaker's icon?
It's not. WindowMaker is actually using ours. The GNUstep yin/yang icon was created by Ayis Theseas Pyrros for GNUstep, not for WindowMaker.
Relation to Window Maker
Window Maker is a window manager, not a workspace manager nor a file browser. It is nothing more. Window Maker and GNUstep share almost no libraries or functionality. Window Maker is written in C, and GNUstep is written in Objective-C. Window Maker does make certain things easier for GNUstep, but it is not GNUstep itself, although it is a part of the project.
The Importance of this difference
In many people's minds the environment and the frameworks used to create programs merges. Window Maker can be used with QT or GNOME apps, just as easily as it can be used with GNUstep. Similarly GNUstep applications can run using KWM (KDE's preferred Window Manager) or Enlightenment (GNOME's old, old preferred Window Manager -- since then they went through Sawfish and now Metacity). This misconception can make one lose sight of the powerful programming frameworks underneath.
Window Maker is an excellent window manager and none of the preceeding should be construed to imply otherwise.