Gold Naxxaramas cards are all crafted.
They are not obtained by beating the bosses on heroic. You can craft them after you get the normal versions of the card by beating Naxxaramas.
Golden versions of cards are not obtainable through play, but become craftable using Arcane Dust once the player has obtained the regular version of that card. Golden Curse of Naxxramas cards cost the usual amount to craft for their rarity.