Why single out GNU? There are a lot of other components of the system that are equally necessary to make it useful. The GNU stuff isn't such a critical thing anymore. We might as well call the system Firefox/KDE/Xorg/Linux or something.
But not just "linux" either. Same as we don't call Mac OS X "Darwin". Linux is just a kernel.
You can run a system without Firefox, KDE or Xorg. But on most systems at least a few GNU apps (at minima a shell) is required around linux.
GNU is the acronym for "Gnu is Not Unix". This means that GNU is designed to provide an OS comparable to Unix, without being exactly Unix. Therefore its name should appear in the OS name, imo.
Firefox or Xorg are not designed to be an operating system. They are just applications.