Now, we have a Microsoft system that runs the Linux binaries, which are GNU according to Stallman, but not running them on the Linux kernel. You’re naming the entire system after the one component that is missing. By the same logic that normal Linux should be GNU/Linux, Linux containers on Windows should be GNU/Windows.
Cygwin allows running GNU software on Windows, but it doesn't implement the Linux ABI. Therefore Cygwin requires software recompilation.
The big advantage of WSL is that you can run native linux binaries.