Extract images ids from gallery shortcode into post_content of the wordpress posts or custom post types.

get_gallery_attachments.php

<?php
/**
 * return $images_id array()
 */
function get_gallery_attachments(){
	global $post;
	
	$post_content = $post->post_content;
	preg_match('/\[gallery.*ids=.(.*).\]/', $post_content, $ids);
	$images_id = explode(",", $ids[1]);
	
	return $images_id;
}