Replace Gutenberg gallery block with the gallery shortcode.

kli_gutenberg_gallery_fix.php

<?php
	/**
	 * WordPress Gutenberg gallery block fix to the gallery shortcode.
   */


	/**
	 * Replace Gutenberg gallery block with the gallery shortcode.
   * Tested until Gutenberg 2.4
   *
	 * @param $the_content
	 *
	 * @return string
	 *
	 */
	if ( ! is_admin() ) {
		add_filter('the_content', function($content) {
			if ( ! is_main_query() ) {
				return $content;
			}
			// Find gallery blocks
			$regexp = "<ul\s[^>]*wp-block-gallery[^>]*>(.*)<\/ul>";
			if ( preg_match_all("/$regexp/imsU", $content, $matches, PREG_PATTERN_ORDER) ) {
				$updated = false;
				foreach($matches[0] as $key => $match) {
					if ( preg_match_all('/\sdata-id="(.*)"/imsU', $match, $match_ids, PREG_PATTERN_ORDER) ) {
						$matches[1][$key] = '[gallery ids="'. implode(',', $match_ids[1]) .'"]';
						$updated = true;
					}
				}
				if ($updated) {
					foreach ($match_ids[1] as $key => $img) {
						$img_full_href = wp_get_attachment_image_src($img, 'full')[0];
						$img_thmb_href = wp_get_attachment_image_src($img, 'medium')[0];
						$content = str_replace('src="'.$img_full_href, 'src="'.$img_thmb_href, $content);
					}
				}
			}
			return $content;
		}, 9);
	}