This code is taken from https://www.advancedcustomfields.com/resources/gallery/ with a little addition to skip the first image. I added a counter and check it to see if it’s the first one.
<?php
$images = get_field('gallery');
if( count($images) > 1 ):
$count = 0;
?>
<ul>
<?php foreach( $images as $image ):
if ($count == 0) {
// skip the first one
continue;
}
$count++;
?>
<li>
<a 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; ?>
