Pyro: "Linux" is used to refer to the entire unix-like OS, but that doesn't make it any more accurate or justifiable. It originally referred to the kernel and much of the rest of the OS orthogonal and not specific to Linux. GNU is a large part of it. KDE (desktop environment) and the X11 windowing system are examples of Operating System software which are specific to neither GNU nor Linux despite being commonly used with both.

The linux kernel is very important, because the GNU project failed to produce a viable kernel. But the GNU project predates Linux and was responsible for the idea, initial implementation and spread of Free Software. Referring to the entire operating system as Linux "can" be done, because it's convenient and seldom ambiguous. Linux really does refer only to the kernel. The name GNU/Linux is more accurate, and more useful e.g. in an article introducing the operating system.

...the thing is that "Linux" on its own is so convenient, and so more commonly used, that in order to promote the name "GNU/Linux" you need a really strong argument. Which there is... but both the argument and its very existence and strength are counterproductive in terms of getting people to take both the OS and the Free Software movement seriously.

IOW: GNU/Linux is a more accurate name for the operating system as a whole. Linux is really just the kernel (and a cloud of linux-specific tools). People can also use it to refer to the operating system as a whole, and I wouldn't say "there's nothing wrong with that", just that it wouldn't be useful to correct it.

a'course, one of the reasons for the lack of a single useful name for the OS is not just the diversity of the individual components. It's the diversity of the OS itself. If Ubuntu was the only Linux based OS, we could just talk about Ubuntu. But Ubuntu is based on another OS, Debian, which is completely different to and separate from Red Hat, etc. The lack of a single agreed name is a very natural consequence of the nature of the thing: Freedom is a synonym of Anarchy.