Make WordPress function Get_Post_Gallery work for Gutenberg powered sites.

get-post-gallery.php

<?php
/**
 * Plugin Name: Get Post Gallery Polyfill
 * Plugin URI: https://prothemedesign.com
 * Description: Make Get_Post_Gallery work for Gutenberg powered sites.
 * Author: Ben Gillbanks
 * Version: 1.0
 * Author URI: https://prothemedesign.com
 *
 * @package ptd
 */

/**
 * A get_post_gallery() polyfill for Gutenberg
 *
 * @param string $gallery The current gallery html that may have already been found (through shortcodes).
 * @param int $post The post id.
 * @return string The gallery html.
 */
function bm_get_post_gallery( $gallery, $post ) {

	// Already found a gallery so lets quit.
	if ( $gallery ) {
		return $gallery;
	}

	// Check the post exists.
	$post = get_post( $post );
	if ( ! $post ) {
		return $gallery;
	}

	// Not using Gutenberg so let's quit.
	if ( ! function_exists( 'has_blocks' ) ) {
		return $gallery;
	}

	// Not using blocks so let's quit.
	if ( ! has_blocks( $post->post_content ) ) {
		return $gallery;
	}

	/**
	 * Search for gallery blocks and then, if found, return the html from the
	 * first gallery block.
	 *
	 * Thanks to Gabor for help with the regex:
	 * https://twitter.com/javorszky/status/1043785500564381696.
	 */
	$pattern = "/<!--\ wp:gallery.*-->([\s\S]*?)<!--\ \/wp:gallery -->/i";
	preg_match_all( $pattern, $post->post_content, $the_galleries );
	// Check a gallery was found and if so change the gallery html.
	if ( ! empty( $the_galleries[1] ) ) {
		$gallery = reset( $the_galleries[1] );
	}

	return $gallery;

}

add_filter( 'get_post_gallery', 'bm_get_post_gallery', 10, 2 );