For a long time before WSL was conceived of, there was a GNU system on top of Windows - cygwin. Cygwin and WSL have entirely different strengths and weaknesses

For a long time before WSL was conceived of, there was a GNU system on top of Windows - cygwin. Cygwin and WSL have entirely different strengths and weaknesses due to the fact that Cygwin is GNU on top of Win32, whereas WSL is GNU on top of a Linux syscall layer implemented by the NT kernel. Earlier versions of Windows had GNU tools on top of the POSIX layer of the Kernel, which was entirely different beast altogether. All "GNU" systems, all very different characteristics. I do think that the GNU/Linux distinction is valid when comparing something like CentOS against Android or even Busybox, but WSL is not a great example because it is almost entirely defined by its implementation of the Linux kernel API.