Caption under product gallery thumbnails

I am looking to add a caption under the product gallery image thumbnail(s). I found this code, which works, but it displays the caption under the large main image:

add_filter('woocommerce_single_product_image_thumbnail_html', function($html, $attachment_id) {
  $caption = get_post_field('post_excerpt', $attachment_id);

  if(trim($caption)) {
    $html = str_replace('</div>', '<span class="gtnCaps">' . $caption . '</span></div>', $html);
  }

  return $html;
}, 10, 2);

How can I tweak this code so the caption displays under the small thumbnail image(s) of the product gallery instead?