Modify the g:ID element in WooCommerce Google Product Feed extension

functions.php

<?php
function lw_woocommerce_gpf_feed_item_google( $feed_item, $product ) {
	// Modify $feed_item->guid here.
	// Note, you may also need to update $feed_item->item_group_id if you have variable products.
	return $feed_item;
}
add_filter( 'woocommerce_gpf_feed_item_google', 'lw_woocommerce_gpf_feed_item_google', 11, 2 );