In order to use GridFS adapter, you should have accesible MongoDB instance and Mongo PHP driver installed.
You can install the Mongo extension with
pecl install mongo
N.B. mongo
php extension is deprecated in favor of MongoDB PHP driver. We will switch to it right after new API supports GridFS.
$client = new \MongoClient('mongodb://localhost:27007');
$db = $client->selectDB('dbname');
$gridFs = new \MongoGridFs($db);
$adapter = new \Gaufrette\Adapter\GridFS($gridFs);