gapi.load('client', () => {
  // now we can use gapi.client
  // ...
});

gapi.client.load(
  'https://gmail.googleapis.com/$discovery/rest?version=v1',
  () => {
    // now we can use:
    // gapi.client.gmail
  }
);

// Deprecated, use discovery document URL, see https://github.com/google/google-api-javascript-client/blob/master/docs/reference.md#----gapiclientloadname----version----callback--
gapi.client.load('gmail', 'v1', () => {
  // now we can use:
  // gapi.client.gmail
});

// declare client_id registered in Google Developers Console
var client_id = '',
  scope = [
    // Read, compose, send, and permanently delete all your email from Gmail
    'https://mail.google.com/',

    // Manage drafts and send emails when you interact with the add-on
    'https://www.googleapis.com/auth/gmail.addons.current.action.compose',

    // View your email messages when you interact with the add-on
    'https://www.googleapis.com/auth/gmail.addons.current.message.action',

    // View your email message metadata when the add-on is running
    'https://www.googleapis.com/auth/gmail.addons.current.message.metadata',

    // View your email messages when the add-on is running
    'https://www.googleapis.com/auth/gmail.addons.current.message.readonly',

    // Manage drafts and send emails
    'https://www.googleapis.com/auth/gmail.compose',

    // Add emails into your Gmail mailbox
    'https://www.googleapis.com/auth/gmail.insert',

    // See and edit your email labels
    'https://www.googleapis.com/auth/gmail.labels',

    // View your email message metadata such as labels and headers, but not the email body
    'https://www.googleapis.com/auth/gmail.metadata',

    // Read, compose, and send emails from your Gmail account
    'https://www.googleapis.com/auth/gmail.modify',

    // View your email messages and settings
    'https://www.googleapis.com/auth/gmail.readonly',

    // Send email on your behalf
    'https://www.googleapis.com/auth/gmail.send',

    // See, edit, create, or change your email settings and filters in Gmail
    'https://www.googleapis.com/auth/gmail.settings.basic',

    // Manage your sensitive mail settings, including who can manage your mail
    'https://www.googleapis.com/auth/gmail.settings.sharing',
  ],
  immediate = true;
// ...

gapi.auth.authorize(
  {client_id: client_id, scope: scope, immediate: immediate},
  authResult => {
    if (authResult && !authResult.error) {
      /* handle successful authorization */
    } else {
      /* handle authorization error */
    }
  }
);

/*
Gets the current user's Gmail profile.
*/
await gapi.client.gmail.users.getProfile({userId: 'userId'});

/*
Stop receiving push notifications for the given user mailbox.
*/
await gapi.client.gmail.users.stop({userId: 'userId'});

/*
Set up or update a push notification watch on the given user mailbox.
*/
await gapi.client.gmail.users.watch({userId: 'userId'});