By default, feedback and grades are available to students as soon as they are entered. If you want to hide feedback and grades on an assignment, just disable the "Show feedback & grades" option at the top right. With this option disabled, students can still submit and critique videos, but they won't see any feedback (comments, markers, etc.) or grades (including rubrics).
With the "Show feedback & grades" option enabled, students will be able to see feedback (according to the assignment privacy settings) and grades (on their submission only).
If you are using GoReact in an LMS (Canvas, Blackboard, etc.), then disabling "Show feedback & grades" will also prevent grades from passing to the LMS grade book. Once "Show feedback & grades" is enabled, all grades will be pushed to your LMS grade book.