Page Generator Pro: Enable Meta Boxes: https://www.wpzinc.com/documentation/page-generator-pro/developers-enable-meta-boxes/

page-generator-pro-enable-meta-boxes.php

<?php
/**
 * Plugin Name: Page Generator Pro: Enable Meta Boxes
 * Plugin URI: http://www.wpzinc.com/plugins/page-generator-pro
 * Version: 1.0.0
 * Author: WP Zinc
 * Author URI: http://www.wpzinc.com
 * Description: Demonstrates how to enable custom and/or third party plugin meta boxes on Page Generator Pro Groups. See: https://www.wpzinc.com/documentation/page-generator-pro/developers-enable-meta-boxes/ 
 */

/**
 * Defines additional meta boxes that are permitted to be displayed on the Generate Content screen.
 *
 * @since 	1.0.0
 *
 * @param 	array 	$permitted_meta_boxes 	 	Array of Meta Box IDs permitted to be displayed on the Generate Content screen.
 * @return 	array 								Array of Meta Box IDs permitted to be displayed on the Generate Content screen.
 */
function page_generator_pro_groups_permitted_meta_boxes( $permitted_meta_boxes ) {

	// Start editing
	// Define the meta boxes that you want to permit to display
	$additional_meta_boxes = array(
		'review_metabox',
	);
	// Stop editing

	// Bail if no additional meta boxes were defined
	if ( empty( $additional_meta_boxes ) ) {
		return $permitted_meta_boxes;
	}

	// Merge arrays and return
	return array_merge( $permitted_meta_boxes, $additional_meta_boxes );

}
add_filter( 'page_generator_pro_groups_ui_permitted_meta_boxes', 'page_generator_pro_groups_permitted_meta_boxes', 10, 1 );

// Backward compat filter
add_filter( 'page_generator_pro_groups_permitted_meta_boxes', 'page_generator_pro_groups_permitted_meta_boxes', 10, 1 );