Gmail add-on basic setup

addon.js

function buildAddOn(e) {
  var accessToken = e.messageMetadata.accessToken;
  GmailApp.setCurrentMessageAccessToken(accessToken);
 
  var textWidget = CardService.newTextParagraph().setText('Hello World')
 
  section.addWidget(textWidget);
 
  var card = CardService.newCardBuilder()
    .setHeader(CardService.newCardHeader()
    .setTitle('Addon Demo'))
    .addSection(section)
    .build();
 
  return [card];
}