How to get gallery images?

I have some images uploaded in wordpress posts (eg. post#1, and post#2). Now I’ve created a new post (eg. Post#3), and in this post I have inserted the WordPress 3.5 gallery, I did not upload any image in the post#3, instead I used the images from the post#1 and post#2.
Now in the post#3, I would like to get the links to the images, but I can not seem to get. I am using the following code:

$attachments = get_children( array('post_parent' => get_the_ID(), 'post_type' => 'attachment', 'post_mime_type' =>'image') );
    foreach ( $attachments as $attachment_id => $attachment ) {
            echo wp_get_attachment_image( $attachment_id, 'medium' );
   }

It will show only the image if I upload in the post, but it won’t show the image if I add from the other posts. So, how can I get the links to the images which are added in the gallery but not uploaded.

Thanks for any help.