@robert_miles , greetings! By "populate" do you mean with students for both or with students for the Inbox and assignments for the Gradebook?
If it's students for both, then you need to have students added to the course. Once students are added to the course, they populate automatically in the Gradebook and Inbox. You can manually add students through the People page in your course - How do I add users to a course?
If you are trying to populate the Gradebook with assignments, then you would want to add the assignments via the "Assignment" page in your course - https://community.canvaslms.com/docs/DOC-9873-415267003
Hope this information is helpful! Kona
