Code snippet to automatically add prefix to product IDs - By WebToffee( Product feed for WooCommerce Basic)

functions.php

<?php // Do not copy this line of code
/**
 * Prefix product ID in feed output.
 *
 * @param string|int $product_id Product ID being processed.
 * @param WC_Product $product    Product object.
 *
 * @return string|int Modified product ID.
 */
function wbtfe_product_feed_alter_product_id( $product_id, $product ) {
	if ( ! empty( $product_id ) ) {
		return 'gla_' . $product_id;
	}

	return $product_id;
}
add_filter( 'wt_feed_filter_product_id', 'wbtfe_product_feed_alter_product_id', 10, 2 );