The gradereport_user_get_grade_items API call returns a categoryid field, but does not return the "full name" of the category. The name associated with the categoryid is found in the mdl_grade_categories table.
Is there an API call that would allow fetching the "full name" associated with the categories? It appears that there is not. But perhaps I have overlooked it?
See associated JIRA ticket https://tracker.moodle.org/browse/MDL-64399.