Had the same query and stumbled on this page.
I found a solution that works perfectly for anyone else that ends up here.
1. Create a new ACF Field Group e.g. “Media Metadata”
2. Add a field as a Link or Page Link or URL (I used Page Link)
3. Then under Location => Rules “show this field group if” = Attachment
4. This will add a new link field into each media library item
(https://www.greengeeks.com/tutorials/article/add-custom-fields-to-media-wordpress/)
I used a Page Link and added links to my Media in the Media Library.
So when calling it in my Gallery loop code:
<?php
foreach( $images as $image ):
$link_id = get_post_meta( $image['id'], 'page_link', true); ?>
<a href="<?php the_permalink( $link_id ); ?>" title="<?php echo $image['alt']; ?>">
<img data-src="<?php echo $image['sizes'][$size]; ?>"/></a>
<?php
endforeach;
?>
