The only -W flag I use that you don’t list here is -Wstrict-prototypes (which warns if you have an empty function prototype, which often means “C++ programmer forgot that () and (void) are different in C prototypes”). But of course, you can’t include that, because gtk.h triggers that warning (and must continue to do so to maintain API compatibility), so you can’t use it with -Werror, which is one of the reasons why -Werror is evil.

(Actually, the real solution for -Werror [and -pedantic/-pedantic-errors] is that they should have a way to only apply it to *your own* code, not to code from system headers. That would also fix the problem where for years on Solaris, it was impossible to compile an X program with -Wall -Werror.)

Warning are beginning to be used more in gnulib, coreutils, bison et. al. now, see:
http://lists.gnu.org/archive/html/bug-gnulib/2008-12/msg00046.html