Chris doesn't know what to do since his Apple GPG tools don't work with macOS Sierra. Leo says that GPG is Gnu Privacy Guard, which is the open source version of Pretty Good Privacy email encryption. You can use any email client with it to encrypt your email. The other side has to be able to decrypt it, though.
GPGTools.org uses a private key to scramble, and a public key to descramble. If it doesn't work with Sierra, Leo says it'll only be a matter of time before it gets updated. There is a workaround using the services menu, so Leo advises going that route.