Hi,elliot.
I couldn’t get result with what I posted.
But finally I could get good result with below code.
Thank you for your comment.
<?php
$gallery = get_field('gallery' ); // get all the rows
$gallery_items = $gallery[0]; // get the first row
$gallery_item = $gallery_items['gallery_photo']; // get the sub field value
if($gallery_item):
?>
<div class="box">
<h3><img src="images/images.png" /></h3>
<div id="gallery" class="ad-gallery">
<div class="ad-image-wrapper"></div>
<div class="ad-nav">
<div class="ad-thumbs">
<ul class="ad-thumb-list">
<?php while(has_sub_field('gallery')): ?>
<?php
$image = wp_get_attachment_image_src(get_sub_field('gallery_photo'), 'large');
$thumb = wp_get_attachment_image_src(get_sub_field('gallery_photo'), 'thumbnail');
?>
<li><a href="<?php echo $image[0] ?>"><img src="<?php echo $thumb[0] ?>"></a></li>
<?php endwhile; ?>
</ul>
</div>
</div>
</div>
</div>
<?php endif; ?>