Photo Gallery looks like a plugin add-on as seen here.
So try adding the code from the plugin example to your template:
<?php
//Get the images ids from the post_metadata
$images = acf_photo_gallery('gallery', $post->ID);
//Check if return array has anything in it
if( count($images) ):
//Cool, we got some data so now let's loop over it
foreach($images as $image):
$id = $image['id']; // The attachment id of the media
$title = $image['title']; //The title
$caption= $image['caption']; //The caption
$full_image_url= $image['full_image_url']; //Full size image url
$full_image_url = acf_photo_gallery_resize_image($full_image_url, 262, 160); //Resized size to 262px width by 160px height image url
$thumbnail_image_url= $image['thumbnail_image_url']; //Get the thumbnail size image url 150px by 150px
$url= $image['url']; //Goto any link when clicked
$target= $image['target']; //Open normal or new tab
$alt = get_field('photo_gallery_alt', $id); //Get the alt which is a extra field (See below how to add extra fields)
$class = get_field('photo_gallery_class', $id); //Get the class which is a extra field (See below how to add extra fields)
?>
<div class="col-xs-6 col-md-3">
<div class="thumbnail">
<?php if( !empty($url) ){ ?><a href="<?php echo $url; ?>" <?php echo ($target == 'true' )? 'target="_blank"': ''; ?>><?php } ?>
<img src="<?php echo $full_image_url; ?>" alt="<?php echo $title; ?>" title="<?php echo $title; ?>">
<?php if( !empty($url) ){ ?></a><?php } ?>
</div>
</div>
<?php endforeach; endif; ?>
