how to show list image in gallery?

I guess you should use get_post_gallery_images function, this is example from documentation below

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' );

https://codex.wordpress.org/Function_Reference/get_post_gallery_images