Mostly, this completions writen by members of Github's "zsh-users" organisation.
But this is list of some authors, not paticipiating there (mostly, co-authors of Gentoo-related completions):

Bapt <bapt@tuxfamily.org>
kaworu <kaworu@kaworu.ch>
David Durrleman <dualmoo@gmail.com>
oberyno <oberyno@gmail.com>
Mamoru Komachi <usata@usata.org>