I think they took the window manager from it to make running gtk apps easier. Admittedly gershwin or similar have a long way to go, but gnustep has the basic design from openstep. There's another similar project but from the ground up.

There's a superficial relationship because XFCE is often configured with a dock-like taskbar, but GNUstep is a GNU clone of Cocoa and the window manager from NeXTSTEP. It tries to mimic early macOS a bit more deeply.