How to retrieve GET parameter in Twig template
You didn't say what template this is for, but app is not defined in any of the templates I have seen. 

What you could do, is add a theme_preprocess_TEMPLATE function to include the GET params. Here is a page template example:

In your mytheme.theme file, add a hook_preprocess_page function.

/**
 * Implements hook_preprocess_page()
 */
function MYTHEME_preprocess_page(&amp;$vars) {
  $vars['get']['name'] = $_GET['name'];
}


Then in your page.html.twig you can render it like this:

&lt;h1&gt;{{ get.name }}&lt;/h1&gt;