Overwrite and extend Gutenberg gallery block. Visit blog post https://bit.ly/34OMvdz

extend_gutenberg_gallery_block.php

<?php

  add_action('init', function() {
    register_block_type('core/gallery', array(
      'render_callback' => function($attributes, $content) {

        $items       = count($attributes['ids']);
        $new_content = "<figure class='wp-block-gallery columns-{$items}'><ul class='blocks-gallery-grid'>";

        foreach($attributes['ids'] as $idx => $attachment_id) {
          
          if (empty($attributes['sizeSlug'])) {
            $attributes['sizeSlug'] = 'full';
          }

          $src    = wp_get_attachment_url($attachment_id);
          $srcset = wp_get_attachment_image_srcset($attachment_id, $attributes['sizeSlug'], null);
          $sizes  = wp_get_attachment_image_sizes($attachment_id, $attributes['sizeSlug'], null);

          $new_content .= "<li class='blocks-gallery-item'><figure><input type='checkbox' name='wp-image-{$attachment_id}' id='wp-image-{$attachment_id}' /><div class='gallery-item-modal'><label for='wp-image-{$attachment_id}'><img src='{$src}' alt='' /></label></div><label for='wp-image-{$attachment_id}'><img src='{$src}' data-id='{$attachment_id}' data-full-url='{$src}' data-link='' srcset='{$srcset}' sizes='{$sizes}' class='wp-image-{$attachment_id}' alt='' /></label></figure></li>";
        }

        $new_content .= '</ul></figure>';

        return $new_content;
      },
    ));
  }, 10, 1);