It would seem that GNU/Linux or Linux (whatever the user-accessing operating system is called) is the only OS that must mention its kernel. No one calls Windows the NT operating system, nor does anyone call Mac OS the Darwin operating system. So why should Linux be the exception?
When I think of GNU, I think of a project that had a very particular goal in mind: build an operating system that replaces Unix with entirely free software. The project got nearly all the way there, but before they got a usable kernel working, Torvalds licensed his kernel with the GPL. With the Linux kernel combined with GNU, we have an OS the GNU project set out to create. So why should Torvalds get all the credit? Without calling the OS GNU, most people don't even know how or why it came to be.
I could see a valid argument to just simply call the OS GNU. It was the name the original team gave the project to have a fully functional OS made with entirely free software. True, Torvalds didn't write Linux for GNU, but neither did the X Window System. A Kernel is essential for operation though, so I can see why the name GNU/Linux was proposed.