Grav+GAnalytics

@emptty0, That is indeed the most simple way.

To add a bit more flexibility you might consider adding test and production GA ID's in different config files, eg. /user/config/ganalytics.yaml containing the test Id and /user/env/mydomain.com/config/ganalytics.yaml containing the production Id.

// /user/config/ganalytics.yaml  -> Contains default setting
script: '<script>script with test key</script>'

// /user/env/mydomain.com/config/ganalytics.yaml
script: '<script>script with production key</script>'

And in /user/themes/your-theme/templates/partials/base.html.twig:

{{ config.ganalytics.script | raw }}

Grav will automatically switch between config files depending on the domain.