_includes/google_analytics.html

<!-- the google_analytics_id gets auto inserted from the config file -->

{% if site.google_analytics %}
<script async src="https://www.googletagmanager.com/gtag/js?id={{site.google_analytics}}"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());
  gtag('config', '{{site.google_analytics}}');
</script>
{% endif %}