Your latest snippet returns this:
$first_row_image = http://stevedowty.com/sunshine/wp-content/uploads/2017/03/mh.jpg
So, this
<?php
$rows = get_field('gallery' ); // get all the rows
$first_row = $rows[0]; // get the first row
$first_row_image = $first_row['gallery_photo' ]; // get the sub field value
?>
<img src="<?php echo $first_row_image; ?>" />
Made the image show up. Unless I’ve done something wrong here that could have been configured better, I’m calling this one done.