Gradebook does not currently log anything in the Sakai event log. The event log is useful both for site statistics (user activity within site, e.g. as reported by the UFP SiteStats tool), and for research purposes when investigating how users have used Sakai, which in turn is important for informing further development.
Gradebook should post events to the Sakai log (gradebook.*) for each meaningful user action, e.g. add an assignment, update grades, etc.
The REF field should preferably include both the site id (context) and a reference to the assignment identifier.