"The OS" doesn't exist. The operating systems you're talking about are called Debian, Ubuntu, Arch, Fedora, RHEL, etc etc. The main work of making an actually usable OS from the various free software components others have written has always been done by the teams responsible for these products.
But we still need a way to refer to them collectively, and it used to make sense to call them "Linux" because they were pretty much the only operating systems that used the Linux kernel, but now that Android is the most widely used OS on the planet, it doesn't anymore, and this alone is a reason to say GNU/Linux unless you want to include Android.