Get raw image detail from post gallery?

A gallery is at heart just the set of attached posts, so…

$post_id = 1; // set to your desired parent post
$attachments = new WP_Query( 
  array(
    'post_parent' => $post_id, // post to which the gallery is attached
    'post_status' => 'inherit', 
    'post_type' => 'attachment', 
    'post_mime_type' => 'image', 
//  'order' => $order, 
//  'orderby' => $orderby
  ) 
);
// var_dump($attachments); // debug

foreach ($attachments->posts as $p) {
  var_dump($p);
  var_dump(wp_get_attachment_image_src($p->ID));
  var_dump(get_post_custom($p->ID));
}

You should be able to find all you need, and a lot more, in that dumped data. In fact, I think that all you need is in the last two of those dumps so you can probably simplify to this:

$post_id = 1; // set to your desired parent post
$attachments = new WP_Query( 
  array(
    'fields' => 'ids',
    'post_parent' => $post_id, // post to which the gallery is attached
    'post_status' => 'inherit', 
    'post_type' => 'attachment', 
    'post_mime_type' => 'image', 
//  'order' => $order, 
//  'orderby' => $orderby
  ) 
);
// var_dump($attachments); 

foreach ($attachments->posts as $ids) {
  var_dump(wp_get_attachment_image_src($ids));
  var_dump(get_post_custom($ids));
}