_includes/google-analytics.html

<!-- 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>