It's based on Debian Bookworm.
Nonetheless, being the maintainer of this project, i'd like to point out that this is no dedicated GNUstep distro, but a mostly Debian based distribution using a thoroughly preconfigured Window Maker as its primary user interface, and that just happens to have the whole range of available GNUstep applications added on.
As already written elsewhere[3], citing my own words, this is a better characterization of its scope:
»Window Maker is just a highly compatible X11 window manager and is supposed to work as such. There is no interest to specifically integrate it with the provided GNUstep applications, as this is not supposed to be predominantly a GNUstep desktop. The included GNUstep applications are just an addon to give people a practical way to verify what GNUstep has to offer. In fact, wmlive would be perfectly usable without providing any single GNUstep application. The freedom and flexibility provided by an X11 window manager instead of the walled garden of a specific desktop system is much more preferable to many Linux users. NeXT nostalgists might want to look elsewhere. [1][2]«
People who's interest has been sufficiently piqued to download wmlive are advised to better wait until after Debian's bookwom 12.12 point release this saturday. A final bookworm based wmlive release will be uploaded shortly after. This will also be the last and final 32bit i386 variant of wmlive. After that work on an exclusively amd64 trixie based wmlive variant will begin.
[1] https://github.com/trunkmaster/nextspace
People back then were looking for something that would be familiar to Windows/Mac users. GNUStep (at least at the time) was not interested in being that.