モーダル画面をHTML記法で

実際のところ

var htmlOutput = HtmlService
    .createHtmlOutput('<p>A change of speed, a change of style...</p>')
    .setWidth(250)
    .setHeight(300);
SpreadsheetApp.getUi().showModelessDialog(htmlOutput, 'My add-on');

参考もと

Class Ui  |  Apps Script  |  Google Developers