For the past 2 years I have helped numerous people debug why their
Debian packaging git repo is messed up, or why imports or builds are
failing on various errors from git-buildpackage not finding branches
or tags or sources.

I would estimate that in 90% of the cases the root cause has been that
the debian/gbp.conf was missing, and thus the different people or the
same person at different times ran git-buildpackage with different
settings, resulting in hard to detect and hard to revert mistakes.

What is git-buildpackage itself would have a small notice on every run
if no debian/gbp.conf was found?

Or even better, if people run it with options like --debian-branch,
--upstream-branch, --upstream-vcs-tag, --pristine-tar etc it could
generate the debian/gbp.conf and tell people that if they commit that
file they don't need to specify those same options again in next
invocation?

I've certainly run into this problem. I used to rely on pristine-tar being
set globally and no debian/gbp.conf in my packages and I accidentally pushed
a non-pristine-tar new upstream version for one of my packages. Now, it's
basically stuck in that state until the next upstream release.

Francois