GAS-ButtonClick.gs

function doGet() {
   var app = UiApp.createApplication();
   var button = app.createButton("Click Me!");
   button.addClickHandler(app.createServerHandler("handlerFunction"));
   app.add(button);
   return app;
}

function handlerFunction(eventInfo) {
  var app = UiApp.getActiveApplication();
  var label2 = app.createLabel("Yo bro, why'd you click me?");
  app.add(label2);
  return app;
}