I am using git to sync portage and have added  the enabling line to
repos.conf:

"sync-git-verify-commit-signature = true"

but only ever get (been enabled for a week now):

* Using keys from /usr/share/openpgp-keys/gentoo-release.asc
 * Refreshing keys from keyserver ...                                   
[ ok ]
 * No valid signature found: unable to verify signature (missing key?)

Is there something else needed?  I do have
app-crypt/openpgp-keys-gentoo-release installed and updated.