adding netlify env variables for google analytics

Gemfile

@@ -1,2 +1,3 @@
 source "https://rubygems.org"
 gem 'github-pages'
+gem 'jekyll-environment-variables'

Gemfile.lock

@@ -114,6 +114,8 @@ GEM
       rouge (~> 2)
     jekyll-default-layout (0.1.4)
       jekyll (~> 3.0)
+    jekyll-environment-variables (1.0.0)
+      jekyll (~> 3.0)
     jekyll-feed (0.11.0)
       jekyll (~> 3.3)
     jekyll-gist (1.5.0)
@@ -243,6 +245,7 @@ PLATFORMS
 
 DEPENDENCIES
   github-pages
+  jekyll-environment-variables
 
 BUNDLED WITH
    1.17.1

_includes/google-analytics.html

@@ -1,11 +1,31 @@
-{% if site.google_analytics_id %}
-  <!-- Global site tag (gtag.js) - Google Analytics -->
-  <script async src="https://www.googletagmanager.com/gtag/js?id={{ site.google_analytics_id }}"></script>
-  <script>
-    window.dataLayer = window.dataLayer || [];
-    function gtag(){dataLayer.push(arguments);}
-    gtag('js', new Date());
+{% if jekyll.environment == "production" %}
 
-    gtag('config', '{{ site.google_analytics_id }}');
-  </script>
+  {% assign gid = site.env.NETLIFY_GOOGLE_ANALYTICS_ID %}
+  {{ gid }}
+
+  {% if gid %}
+    <script async src="https://www.googletagmanager.com/gtag/js?id={{ gid }}"></script>
+    <script>
+      window.dataLayer = window.dataLayer || [];
+
+      function gtag() {
+        dataLayer.push(arguments);
+      }
+      gtag('js', new Date());
+      gtag('config', '{{ $gid }}');
+    </script>
+  {% else %}
+    {% if site.google_analytics_id %}
+      <script async src="https://www.googletagmanager.com/gtag/js?id={{ site.google_analytics_id }}"></script>
+      <script>
+        window.dataLayer = window.dataLayer || [];
+
+        function gtag() {
+          dataLayer.push(arguments);
+        }
+        gtag('js', new Date());
+        gtag('config', '{{ site.google_analytics_id }}');
+      </script>
+    {% endif %}
+  {% endif %}
 {% endif %}