function turnOnAutoResponder() {
  const ONE_DAY_MILLIS = 24 * 60 * 60 * 1000;
  const currentTime = (new Date()).getTime();
  Gmail.Users.Settings.updateVacation({
    enableAutoReply: true,
    responseSubject: 'I am out of the office today',
    responseBodyHtml: 'I am out of the office today; will be back on the next business day.<br><br><i>Created by OOO Chat app!</i>',
    restrictToContacts: true,
    restrictToDomain: true,
    startTime: currentTime,
    endTime: currentTime + ONE_DAY_MILLIS
  }, 'me');
}
Turns on the user's vacation response for today in Gmail.