Accusations, accusations, always accusations

[Richard Stallman]

    Is what is "part of GNU" and what is simply "GNU-compatible" defined
    explicitly somewhere?

GNU is the name of an operating system.  (This is what the GNU Project
set out to develop.)  Something is part of GNU if it is part of that
system.

"GNU-compatible" is not a term I use, so I don't have a
definition for it.

    http://www.gnu.org/software/software.html disclaims completeness.

That page includes the *list* of GNU software packages.  But the
general definition is, a GNU package is a program that is released
under the aegis of the GNU Project.

See http://www.gnu.org/philosophy/categories.html for a list
of definitions of categories of software.