Yii2 multple file upload saving in DB issue

I created a sample multiple file upload functionality to upload gallery images...I followed this method.Every thing is working fine..The image saves in the directory also good.but my problem is when I select multiple images to save in DB,It only saves single image name and I want to save all image name.How Can I rectify this error.
Can any body help me???
MY controller

public function actionCreate()
{
    $model = new Gallery();


    if ($model->load(Yii::$app->request->post()) && $model->save()) {
         $model->gallery_image = UploadedFile::getInstances($model, 'gallery_image');

         if ($model->upload()) {

            // file is uploaded successfully
        }

     return $this->redirect(['view', 'id' => $model->gallery_id]);

    } else {
        return $this->render('create', [
            'model' => $model,
        ]);
    }
}  

my Model

 public function upload()
{

      $gal_id = $this->gallery_parent_id; 

    if ($this->validate()) { 

         $destination = $this->upload_dir.'/gallery/'.$gal_id;
         if(!is_dir($destination))
            mkdir($destination);

                   foreach ($this->gallery_image as $file) {
    $ext = end((explode(".", $file)));
      $filename = Yii::$app->security->generateRandomString().".{$ext}";

            $file->saveAs('images/apartment/gallery/'.$gal_id.'/'. $filename);
          //  $file->save();
             $this->gallery_image = ''.$filename;
                $this->save();


        }
             return true;


    } else {
        return false;
    }


}