Add sale status to custom label in Google Product Feed

functions.php

<?php

add_filter( 'woocommerce_gpf_feed_item_google', function ( $feed_item, $wc_product ) {
	$feed_item->additional_elements['custom_label_4'] = [
		$wc_product->is_on_sale() ? 'On Sale' : ''
	];
	return $feed_item;
}, 10, 2 );