// post ID in second arg
// false in 3rd arg gets the unformatted value
// this will return an array in the order that the images
// are in the gallery
$gallery = get_field('your-gallery-field', $post_id, false);
// use this array to query the attachments
$query = new WP_Query(array(
// other query arguments
'post__in' => $gallery,
'orderby' => 'post__in'
));