FIND:

$form[ 'gallery' ][ 'gallerytype' ][ 'hide_gallery_info' ]   = array( 'type' => 'st--logic-radio' , 'label' => __( 'Hide left sidebar on gallery page' , 'cosmotheme' ), 'hint' =>__( 'If enabled, gallery info will be hidden ' , 'cosmotheme' ), 'cvalue' => options::get_value( 'blog_post' , 'hide_gallery_info' )  );

  ADD BELOW

    $form['gallery'  ][ 'gallerytype' ][ 'show_collapse_btn']    = array( 'type' => 'st--logic-radio' , 'label' => __( 'Show collapse button' , 'cosmotheme' ));