As stated above, the Scrivito Example App blocks all cookies by default. In order to get GTM working, you will also need to enable the built-in cookie consent component. For this, just add a URL to your privacy policy in the “Cookie consent link” attribute in the site settings of your homepage (which can be found in its page properties). With this enabled, and acceptance of cookies by your visitors, tracking data is allowed.
If you have taken steps to prevent your website from being accessed by visitors in the EU or other locals with requirements, removing the component from “src/Components” and not importing it in “src/App.js” is sufficient.