I have found that the "Add a block" block on the gradebook page is controlled by the editing setting of the course main page. If you have editing turned on on the course page, Add a block shows up on the gradebook page. If you turn editing off on the course main page, there is no Add a block on the gradebook page. This seems to be because Turn editing on on the gradebook page controls editing the grades themselves, not the page.
I hope that helps.
Wendell