Something like this should do what you need:
<?php
$images = get_field('section_gallery');
if( $images ): ?>
<?php foreach( $images as $image ): ?>
<div>
<a data-fancybox="gallery" href="<?php echo $image['sizes']['large']; ?>">
<img src="<?php echo $image['sizes']['medium']; ?>" alt="<?php echo $image['alt']; ?>">
<?php if ($image['caption']) { ?><span><?php echo $image['caption']; ?></span><?php } ?>
</a>
</div>
<?php endforeach; ?>
<?php endif; ?>
