Realistically GNUStep is not ready to replace OSX by a long way. I was experimenting with doing so for a long time, but there hasn't been enough changes to the core components of the system yet, a lot more focus is on API features than application features. (System Preferences exists but lacks everything useful: Screen Resolution Changing/multimonitor/Wireless/Networking(Network manager or equivalent, wrapped)/Sound(Pulseaudio Wrapper)).
The GWorkspace application is quite good, but could be tweaked to be more like the Mac Finder. It already has everything you really need.
The big dealbreaker for me is Vespucci.app, realistically a web browser is critical to using gnustep on a daily basis, and right now it just doesn't have one. There used to be Mantella under Etoile which wrapped Firefox into a gnustep window which was pretty decent. But the subsystem of firefox it used has been deprecated so the code no longer works.
A multi-tabbed terminal.app would also fix a lot of the frustration in using GNUStep as a standalone desktop system.
I think GNUStep is going to remain a useful tool for porting Mac apps to Linux/other platforms, but outside of that, is going to remain very niche as a desktop system because it hasn't got the desktop system in place yet. Also, I know that my above post looks like I favor Linux as the underlying system giving examples such as PulseAudio/Network Manager, but I would actually prefer if the System Preferences area was able to wrap Windows/Mac and Linux systems equally.