<?php
require_once(MODX_CORE_PATH.'components/gallery/processors/mgr/item/getlist.class.php');

class modSiteGalleryProcessor extends GalleryItemGetListProcessor{

public function outputArray(array $array, $count = false){
        return array(
            'success' => true,
            'message' => $this->getMessage(),
            'count'   => count($array),
            'total'   => $count,
            'object'  => $array,
        );
}
}
return 'modSiteGalleryProcessor';

<?php
$response = $modx->runProcessor('gallery/getlist', 
array(
    'album' => 1,
), 
array(
    'processors_path' => $modx->getOption('core_path') . 'site/processors/'
));
print_r($response->response);