This Google Script will encrypt Gmail using the powerful and secure AES encryption. It then converts the JSON response to Base64 and sends the encrypted message via GMailApp service. See video tutorial.

The password for encryption is supplied via the input field in the form. The recipient will need this password to decrypt messages.

function encryptMail(e) {
  try {
    // get the first message from the drafts folder in Gmail
    var draft = GmailApp.search('in:drafts')[0].getMessages()[0];

    // retrieve the email body in plain text
    // ( you can also do getBody() for HTML Mail )
    var body = draft.getPlainBody();
    var to = draft.getTo();
    var subject = draft.getSubject();

    // The encrypted message is in JSON format
    var json = sjcl.encrypt(e.parameter.password, body);
    // Convert the JSON to base64 (easier to copy-paste)
    var msg = Utilities.base64Encode(Utilities.jsonStringify(json));

    // Send the encrypted message via Gmail
    GmailApp.sendEmail(to, subject, msg);
  } catch (e) {
    Logger.log(e.toString());
  }
}