They are different and not mutually exclusive things. Window Maker is a window manager with a NeXTStep-like look. GNUstep is an application framework. You can

They are different and not mutually exclusive things. Window Maker is a window manager with a NeXTStep-like look. GNUstep is an application framework. You can use GNUstep with Window Maker (and many do) but you can also use it with KDE, GNOME, XFCE, FVWM, IceWM, JWM or whatever window manager you like. AFAIK GNUstep also has a Wayland backend so you could also use it with Wayland compositors. Similarly you do not have to use Window Maker with GNUstep or any ObjC runtime or libraries, just use it by itself as a window manager. Personally i do not use GNUstep at all but i use Window Maker. (note that Window Maker does create/use a GNUstep directory in the home directory but that is a historical artifact and IIRC in recent versions it can be made to use a directory inside .config instead).