get_post_gallery_images()

function get_post_gallery_images( $post = 0 ) {
	$gallery = get_post_gallery( $post, false );
	return empty( $gallery['src'] ) ? array() : $gallery['src'];
}

 add_filter( 'the_content', 'gcdocs_show_gallery_image_urls' );

/**
 * Show image URLs below the content
 */
function gcdocs_show_gallery_image_urls( $content ) {

	global $post;

	// Only do this on singular items
	if( ! is_singular() )
		return $content;

	// Make sure the post has a gallery in it
	if( ! has_shortcode( $post->post_content, 'gallery' ) )
		return $content;

	// Retrieve the first gallery in the post
	$gallery = get_post_gallery_images( $post );
	$image_list = '<ul>';

	// Loop through each image in each gallery
	foreach( $gallery as $image_url ) {
		$image_list .= '<li>' . '<img src="' . $image_url . '">' . '</li>';
	}
	$image_list .= '</ul>';

	// Append our image list to the content of our post
	$content .= $image_list;

	return $content;
 }