I've mentioned this a few times but never publicly announced it, so consider this the announcement.
I've also ported Owl, my Cocoa Wayland compositor, from OS X to the Hurd using GNUstep.
Here's a screenshot of weston-terminal and weston-flower, running on Owl on GNUstep on Hurd, with X forwarded from a QEMU VM via SSH.
I think you're misunderstanding my post. I did not write a Wayland backend for GNUstep; I kind of did the reverse, I wrote a GNUstep backend "for Wayland". Namely, Owl is a Wayland compositor/server and a Cocoa app; it renders its clients' windows that it receives over Wayland using Cocoa as its backend (a lot like XQuartz does for X11).
The Cocoa implementation (here, GNUstep) then of course uses a backend of its own to render; here it's X11.
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