This is known as array dereferencing and is only available in PHP 5.4+
To support older versions, you need to assign the array, and then access the index:
$data = get_post_meta( $product_id, '_product_image_gallery' );
$gallery_pictures_id = $data[0];
Having said that, in your case use the third argument “single”:
$gallery_pictures_id = get_post_meta( $product_id, '_product_image_gallery', true );