Access the Credentials section of the project at Google Cloud Console.
Click "OAuth consent screen".
Click "Save" button.
In general, the ability to "manage" (and not just "view") your GMail includes the various actions that you might perform manually when interacting with your account via the standard web-interface. For example, such actions can include deleting, starring, labeling, and archiving individual emails -- to name just a few.
I personally do not use Slice.com, and ...