CB Gallery privacy on a per item basis is controlled by the "CB Gallery - Privacy - Edit", "CB Gallery - Privacy - Save", and "CB Gallery - Privacy - Authorized" system actions. To change the default privacy rule you need to either change it globally or make a copy of those system actions, unpublish the system actions, then change the default in your copy. In a future release this will all just be moved to be a part of CB Gallery instead of being in CB Auto Actions.