How do I add an extra signing cert to an existing gem?
Presumably you self-issued your own cert, and it's complaining that the cert chain is broken as a result. The gem's owner should issue your cert using their own as the certification authority.
https://support.dnsimple.com/articles/what-is-ssl-certificate-chain/
It can be done using openssl. The docs for doing so are - unfortunately - abysmally bad and the steps you need to take to do so are hairy - to put it mildly. Possible walkthrough here.