It’s not currently possible to show an ACF Gallery inside Oxygen’s Repeater if the Repeater data source is an ACF Repeater. However, there is a workaround using a Code Block element and Slick carousel.

First, download the slickGallery ZIP file, extract the “gallery” directory and place it in your site root.

Next, add a Code Block element ABOVE your Repeater. You will need to add CSS, JS and PHP to the associated Code Block tabs.

/*
* Add this to the CSS tab of the Code Block ABOVE the Repeater:
*/

.thumbnail img {
   box-sizing: border-box;
   border: 1px solid #ddd;
   padding: 2px;
   margin: 0 1% 15px 0;
   width: 32.6667%;
   display: inline-block;
}
	
.thumbnail img:nth-of-type(3n+3) {
   margin-right: 0;
}

// Add this to the JavaScript tab of the Code Block ABOVE the Repeater:

jQuery(document).ready(function(){ 
  jQuery('.gallery').slickLightbox({ 
      itemSelector: '> a' 
   }); 
});

// Add this to the PHP & HTML tab of a Code Block ABOVE the Repeater:

<?php
  session_start();
  $_SESSION['row_index'] = 0;
?>

Then, add a Code Block element inside your Repeater. You need to place it wherever you want your ACF Gallery to show. Add the following code to the PHP & HTML tab of the Code Block and adjust the ACF field names on line #6 and #10 as needed.

// Add this to the PHP & HTML tab of a Code Block INSIDE the Repeater. Adjust the field names as needed.

<?php

  // This is your Repeater field name
  $rows = get_field('elements-repeater' );
  $specific_row = $rows[$_SESSION['row_index']];

  // This is the Gallery field name
  $gallery_images = $specific_row['gallery_of_photos'];

if($gallery_images): ?> 

   <div class="gallery"> 

      <?php foreach( $gallery_images as $image ): ?> 

         <a href="<?php echo $image['url']; ?>" target="_blank" class="thumbnail"> 
            <img src="<?php echo $image['sizes']['medium']; ?>" alt="<?php the_title(); ?>" /> 
         </a> 

      <?php endforeach; ?> 

   </div> 

<?php endif; ?>


<?php    

  $_SESSION['row_index'] ++;

?>

Finally, install the Code Snippets plugin and create a snippet with the following code to enqueue the Slick files added to the site root in the first step:

Add this to the Code Snippets plugin: https://en-gb.wordpress.org/plugins/code-snippets/

<?php

add_action( 'wp_enqueue_scripts', 'enqueue_slick_files' );
/**
 * Loads <list assets here>.
 */
function enqueue_slick_files() {

    // Enqueue CSS
    wp_enqueue_style( 'slick-css', '/gallery/slick.css' );
    wp_enqueue_style( 'slick-theme', '/gallery/slick-theme.css' );
    wp_enqueue_style( 'slick-lightbox', '/gallery/slick-lightbox.css' );

    // Enqueue JS
    wp_enqueue_script( 'slick-js', '/gallery/slick.min.js', '', '', true );
    wp_enqueue_script( 'slick-lightbox-js', '/gallery/slick-lightbox.js', '', '', true );

}

?>

That should be everything you need to get your ACF Gallery working in the Repeater.