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() || ! strpos($content, 'wp-block-gallery') ) {
		        return $content;
		    }

		    // Find gallery blocks
		  	$regexp = "<ul\s[^>]*wp-block-gallery[^>]*>(.*)<\/ul>";
		  	if (preg_match_all("/$regexp/imsU", $content, $matches, PREG_PATTERN_ORDER)) {
				foreach($matches[0] as $key => $match) {
					// Find images ids
				  	if ( preg_match_all('/$\sdata-id="(.*)"/imsU', $match, $match_ids, PREG_PATTERN_ORDER) ) {
				 		$matches[1][$key] = do_shortcode( '[gallery ids="'. implode(',', $match_ids[1]) .'"]' );
				  	}
			    }		    
			    $content = str_replace($matches[0], $matches[1], $content);
		  	}
			return $content;
		}, 9);
	}