I found this here and am not sure how to work it into the code you posted above, but it looks to be a good way to get it to link to the image itself in the fancybox/lightbox.
<?php
$images = get_field('gallery');
if( $images ):
?>
<ul>
<?php foreach( $images as $image ): ?>
<li>
<a class="fancybox" href="<?php echo $image['url']; ?>">
<img src="<?php echo $image['sizes']['thumbnail']; ?>" alt="<?php echo $image['alt']; ?>" />
</a>
<p><?php echo $image['caption']; ?></p>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?>
PS: Edit the above HTML as needed. 🙂
