Assigning to Robin for triaging, he has some context. See https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=8696623f2b2bda34437e4ebe211f458af437299d
I agree it makes sense to enforce devs using their @gentoo.org address, but we should also consider proxied commits where the author will never have such an address. Such attribution is important - see https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=55930580cbadf421a65488447e45c78304230a72 for example.
This is part of GLEP 63 which is not being enforced yet. And sadly not very many devs have made new keys that do meet the glep. Some have as well as some that have fixed their existing key to meet the glep. Last I heard, Robin still had not gotten the hardware to hold infra gpg keys securely. Once that was done, we were going to give a deadline to bring their gpg keys up to glep spec for acceptance.
repoman support has been removed per bug 835013. Please file a new bug (or, I suppose, reopen this one) if you feel this check is still applicable to pkgcheck and doesn't already exist.