We have just released version 1.0.0 of our Google Tag Manager app. Add this app to your site, and it will add the snippet automatically in the right place in your HTML page (start of body element).
The only configuration required is adding the container ID.
Note that this is achieved with the new Response Filters introduced in Enonic XP 6.2.0.
GitHub repo: GitHub - enonic/app-google-tag-manager
App download: http://repo.enonic.com/public/com/enonic/app/google.tagmanager/
