hack the woocommerce ajax reload of the order review.

gistfile1.php

remove_action('woocommerce_checkout_order_review', 'woocommerce_order_review');

add_action('woocommerce_checkout_order_review', 'cu_pr_update_order_review', 10);

function cu_pr_update_order_review() {
  ob_start();
	global $woocommerce;

	woocommerce_get_template( 'checkout/review-order.php', array( 'checkout' => $woocommerce->checkout() ) );
	$table = ob_get_contents();
	ob_end_clean();
	
	$modified = preg_replace('#<span class="amount">.*</span>#', '<span class="amount">Gratis</span>', $table);
	echo $modified;
}