https://gpgtools.org/ GPGtools. If you use a mac, it can hook into your native mail app and encrypt/sign things with your private key. Sure, whatever provider you use knows you send or receive encrypted mails, and the metadata will always be unencrypted, but these are always the case regardless of email provider.

There seem to be other GPG mail tools out there for different contexts (maybe one that runs in the gmail web interface?) too.