добавил вызов gtag через window.onload

apps/course/templates/course/course.html

@@ -395,7 +395,7 @@
 {% block foot %}
 {% include "templates/blocks/popup_course_buy.html" %}
 <script type="text/javascript" src="{% static 'course.js' %}"></script>
-<script defer>
+<script>
 
   console.log('{{ course.id }}');
   console.log('{{ course.title }}');
@@ -403,6 +403,7 @@
   console.log('{{ course.category.title }}');
   console.log('{{ course.price }}');
 
+  window.onload = function () {
     gtag('event', 'view_item', {
     "items": [
       {
@@ -414,6 +415,7 @@
         "price": "{{ course.price }}"
       }
     ]
-});
+  })
+  };
 </script>
 {% endblock foot %}