GNU does have a usable kernel, it's called Linux.
GNU is the name of the operating system. When Linux is used as its kernel,
Linux is a part of the GNU operating system.
The name of the operating system was coined in 1984, following the old
tradition of naming that gives a nod to its predecessor. Here, Unix is the
predecessor, hence the letter U in the the acronym. GNU stands for Gnu's Not
Unix.
Unix is an operating system. GNU is a replacent for Unix. Therefore, GNU is
an operating system.
The kernel Linux was added to GNU in the early 90s.
If a soldier named Gilbert loses a limb and gets a prosthetic replacement
called Limbux added to his body, it would be incorrect to call him Limbux
(unless he decided to change his name). He's still Gilbert, despite his
prosthetic Limbux.
