Example to add extra files to the zip archive. The file is added to all folder inside the Archive.

bulk-download-for-gravity-forms-add-extra-files-to-single-entry.php

<?php
/**
 * Plugin Name: Bulk Download for Gravity Forms - Add extra files to zip archive.
 * Description: Example to add extra files to the zip archive. The file is added to all folder inside the Archive.
 * Author: VCAT Consulting GmbH
 * Author URI: https://www.vcat.de
 */

/**
 * Filter to add extra files into a single entry.
 *
 * @param array $uploaded_files All uploaded files.
 * @param int   $entry_id The entry ID .
 * @param array $form The form array.
 *
 * @return array
 */
function my_bdfgf_extra_files_single_entry( $uploaded_files, $entry_id, $form ) {
	/*
	 * Add extra file to the zip archive.
	 * Extra file must be on the server!
	 *
	 */

	$wp_upload_dir = wp_upload_dir();

	$uploaded_files[$entry_id][] = $wp_upload_dir['basedir'] .'/extra_files/Information.pdf';
	return $uploaded_files;
}

add_filter( 'bdfgf_single_entry_uploaded_files', 'my_bdfgf_extra_files_single_entry' , 10 , 3);