maybe that work. (when my interpretation of that sample was correct)
<?php
$images = get_field('gallery');
$img_count=1;
if( $images ): ?>
<?php foreach( $images as $image ):
if ($img_count == 1) { ?>
<div id="imageview">
<img src="<?php echo $image['sizes']['large']; ?>" alt="<?php echo $image['alt']; ?>" />
</div>
<div id="thumbnails">
<a class="active" href="<?php echo $image['url']; ?>">
<img src="<?php echo $image['sizes']['thumbnail']; ?>" alt="<?php echo $image['alt']; ?>" />
</a>
<?php } else {?>
<a href="<?php echo $image['url']; ?>">
<img src="<?php echo $image['sizes']['thumbnail']; ?>" alt="<?php echo $image['alt']; ?>" />
</a>
<?php }
$img_count++; ?>
<?php endforeach; ?>
</div>
<script type="text/javascript">
jQuery(document).ready(function() {
$('#thumbnails').simplethumbs({slideshow: '#imageview'});
});
</script>
<?php endif; ?>
probably you also need to load additional simplethumbs js somewhere.
test it and give feedback
