hacker-gh-pages-theme/_includes/head-custom-google-analytics.html

{% if site.google_analytics %}
<!-- Google tag (gtag.js) -->
<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 %}