NextSpace is an attempt to build a full OS around GNUstep: https://github.com/trunkmaster/nextspace

The person behind it seems to still be plugging away. I’d love to see some real inertia behind this kind of effort.

GNUstep is Cocoa, not the OS. OPENSTEP the OS (aka NeXTSTEP but with new OpenStep APIs) and OpenStep the thing that became Cocoa are frustratingly named.