I think you're grammatically correct, Pete, but "Christus natus est" doesn't fit the meter. Let's chalk that one up to "poetic license." Medieval and ecclesiastical Latin took a lot of license, and "Gaudete" was written so late in the medieval-ecclesiastical period that it was far removed from the grammar of Cicero. You'll find much better grammar in 20th-century Latin, such as that used during the Second Vatican Council - but the meter of medieval and ecclesiastical Latin is much more suitable for music. -Joe Offer-