Are you interested in add mypy/PEP-484 annotations to gunicorn?
I have no objection. It's an accepted PEP after all! I cannot speak for everyone, though.
If we do this, we should do it in the comment style so that we maintain Python 2 compatibility.
what would be the benefits for the project?
It functions as a form of documentation and mypy/pytype/pylint either support it or have plans to. It may help contributors this way. It also might work to gently discourage confusing behavior like functions returning multiple types depending on the code path, which I find more error prone and difficult to reason about.
Mostly, I see it as having no negative effect at all, and maybe some gentle positives.