<?php
function lw_woocommerce_gpf_feed_item_google( $feed_item, $product ) {
	// Modify $feed_item->purchase_link here.
	return $feed_item;
}
add_filter( 'woocommerce_gpf_feed_item_google', 'lw_woocommerce_gpf_feed_item_google', 10, 2 );