If this is my misreading, please delete this thread and accept my apologies.
But
https://www.advancedcustomfields.com/resources/gallery/
mentions a return format which is not in the admin
so the field returns an array.
making this template usage wrong;
<?php
$images = get_field('gallery');
$size = 'full'; // (thumbnail, medium, large, full or custom size)
if( $images ): ?>
<ul>
<?php foreach( $images as $image_id ): ?>
<li>
<?php echo wp_get_attachment_image( $image_id, $size ); ?>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?>
it should read
<?php
$images = get_field('gallery');
$size = 'full'; // (thumbnail, medium, large, full or custom size)
if( $images ): ?>
<ul>
<?php foreach( $images as $image ): ?>
<li>
<?php echo wp_get_attachment_image( $image['id'], $size ); ?>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?>
