gantt-period.html

<div class="btn-group mt-1 mx-auto">
  <button id="gantt-day" type="button" class="gantt-period btn btn-sm btn-secondary">Day</button>
  <button id="gantt-week" type="button" class="gantt-period btn btn-sm btn-secondary">Week</button>
  <button id="gantt-month" type="button" class="gantt-period btn btn-sm btn-secondary">Month</button>
</div>

<script>
  document.querySelectorAll(".gantt-period").forEach(function(button) {
    button.addEventListener("click", function(event) {
      railsGantt.gantt.change_view_mode(event.currentTarget.innerHTML);
      document.querySelectorAll(".gantt-period").forEach(function(button) {
        button.classList.remove("active");
      });
      event.currentTarget.classList.add("active");
    });
  });
</script>