Ah, interesting. I didn’t know about unformulated values and the false parameter. Thank you.
For anyone seeing/needing this later, here is the final working code (just remove 'option', if you’re not using a gallery from an Options page):
<?php
$images = get_field('gallery', 'option', false);
$size = 'full'; // (thumbnail, medium, large, full or custom size)
$rand = array_rand($images, 1);
if( $images ): ?>
<?php echo wp_get_attachment_image( $images[$rand], $size ); ?>
<?php endif; ?>
