Fair point, but I was responding to "GNU runs on many OSs", saying that GNU is actually part of the OSs (and not something that runs on top of it), regardless of how much GNU there is in any particular OS.
That's not really true, in broad strokes. To my knowledge, Linux distributions are the only place where GNU forms of core part of the operating system, whereas it is frequently installed as an optional component on top of, say, Solaris, AIX, or a BSD. Termux, amusingly enough, marks a return to this on top of Android on top of Linux.
That's missing my point. What I mean is that where GNU tools are installed, they are a constituent part of the operating system as opposed to running on top/despite the OS. I never said it runs on all operating systems or on all *NIX operating systems.