Mark field as able to prepopulate

functions.php

<?php
/*
 * Plugin Name: Mark GPF fields as pre-populatable
 * Plugin URI: http://docs.woothemes.com/document/google-product-feed/
 * Description: Example of how to mark a field as pre-populatable in the Google Product Feed plugin.
 * Version: 1.0
 * Author: Ademti Software
 * Author URI: https://www.ademti-software.co.uk/
*/

add_filter( 'woocommerce_gpf_all_product_fields', function ( $fields ) {
	$fields['energy_efficiency_class']['can_prepopulate'] = true;
	return $fields;
} );