Issue on Getting Images URL of the Post Gallery

I am trying to get the URL of the attached galley into the post on custom page layout called Gallery Page. I tried to use the code from Codex at This Page (get post gallery images). but I am not getting any thing on the page!

Can you please let me know what I am doing wrong?

<?php
/**
 * Template Name: Gallery Page
 *
 */

 if ( have_posts() ) {
    while ( have_posts() ) {
        the_post(); 
        function pw_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;

 }
 add_filter( 'the_content', 'pw_show_gallery_image_urls' );
    } // end while
} // end if
?>