var message = 'Subject: Test mail';
  var data = {
    userId: 'me',
    message: {
      raw: new Buffer(message).toString('base64')
    },
    media: {
      mimeType: 'message/rfc822',
      body: 'Hi'
    }
  };
  gmail.users.drafts.create(data ,function(err, result) {
    if (err) return res.send(err);
    return res.send(result);
  });

Hi
Date: Fri, 17 Apr 2015 09:21:53 -0700
Message-Id: <CACbdow3+s857hRrMGnqZYxqNgm5CWxfd_tPw8bn07vp+BrLGeg@mail.gmail.com>
From: xxxxx@gmail.com

var message = "To: receiver@gmail.comn" +
              "From: sender@gmail.comn" +
              "Subject: Subject Textnn" +

              "The actual message text goes here";

var data = {
  userId: 'me',
  media: {
    mimeType: 'message/rfc822',
    body: message
  }
};

gmail.users.drafts.create(data ,function(err, result) {
  if (err) {
    res.send(err);
  } else {
    return res.send(result);
  }
});