"The GNUstep project does not provide a window manager; you can use any window manager you want, although we recommend that you use WindowMaker for a better experience." http://gnustep.org/information/aboutGNUstep.html :)

GNUstep is really a framework and closer to something like Gtk+ or Qt. Window Maker is a window manager, it has its own GUI toolkit (WINGs) but it is really only meant for use with Window Maker and perhaps some dockapps (though some dockapps use other toolkits anyway).

Window Maker is an X11 window manager with a NeXTStep look.

GNUStep is an attempt at creating a Free/Libre NeXTStep-like environment, including all the apps.

It doesn't. They're different types of tools. GNUstep commonly runs under and uses Window Maker.

Window Maker is a window manager.

GNUstep is a large set of libraries and foundation classes, and almost as a byproduct, they happen to implemented most of a desktop environment: a file manager and actual desktop, a dock, plain and rich text editors, a pretty good email client, a chat client, a terminal emulator, image viewer, etc. etc.

But like any other xNix desktop, it needs a window manager. KDE uses kwm, GNOME uses mutter, Xfce uses xfwm, LXDE uses OpenBox, etc. GNUstep can use whatever you want, but Window Maker is the recommended choice because it matches the look-and-feel nicely.