Do not allow Private Galleries using Bp Gallery

add_filter( 'gallery_get_valid_status', 'bpcustom_filter_gallery_status', 100 );
function bpcustom_filter_gallery_status( $stati ){
    //do not allow private
    unset($stati['private']);//you can do the same for public, friendsonly too

    return $stati;

}

#bp-gallery