This app will add the Google Tag Manager snippet to your site.
https://market.enonic.com/vendors/enonic/com.enonic.app.google.tagmanager
This app can be downloaded from our repo, or you can download the source code and build the app yourself.
Just add the app to your site. The only configuration required is setting the Container ID for Google Tag Manager.
The snippet will only be added to your site in "live" mode. In other words, when you edit or preview your site, Google Tag Manager will not be enabled.