For the general user (who would use Windows otherwise):
Yes, windowing system is included.
Yes, a desktop environment is included.
Also add: a web browser, a mail client, an office suite, a multimedia player.
Useless: development tools, emacs, many many command line tools (awk, sed, ..).
With this setup (using the KDE desktop) the remaining of the GNU operating system are minimal. Much MUCH less than 8%.
We should be grateful for the GNU project and how it created the free software movement.
But the marginality of GNU in such a setup and the fact that often
FSF/RMS positions are "unpleasant" (copyright assignment, GFDL,
GPL V3 vs. V2, the Gnome project) are the main reasons why calling the
result GNU/Linux is wrong.