The Gamepedia article on Hearthstone card rarity explains gold cards this way:
Cards within the 4 basic rarities described above can have a chance of being obtained with a gold border with custom art animation. Gold cards do not appear frequently, and therefore add to the rarity of a card.
Gold cards can also be crafted with Arcane Dust for ten/eight/four/two times the normal cost of the card (in order of rarity).
Gold Cards are commonly compared with "foil" cards of other types of Trading Card Games (TCGs).
So basically, expert set gold cards drop randomly when opening packs or can be crafted. Additionally as indicated in this question and answer basic gold cards (which cannot be crafted) are earned by leveling heroes.
No, you'll just earn 6 cards (1 legendary and 5 'commons').
Cards you'll earn :
Haunted Creeper good for a zoo deck (it's a deck with low cost cards and very agressive) you'll earn this if you beat Anub'Rekhan in normal.
Nerub'ar Weblord good for counter battlecry, you'll earn this if you beat Faerlina in normal.
Nerubian Egg it'll combo with a solfurie or an Argus, because if you destroy it, you'll earn a nerubian (it's a 4/4 cards), you'll earn this if you beat Maexxna in normal.
Maexxna she destroy any card she attacked, it can be usefull if you play Anduin Wrynn because he can heal Maexxna. You'll earn this if you beat all the 3 boss in normal and you'll unlock heroic and druid and rogue class challenges.
Poison Seeds : destroy all monsters on the battlefield and give treants 2/2 for each destroyed monster. Can combo with a druid token. Unlock when you finish the druid class challenge.
Anub'ar Ambusher a 5/5 for 4 mana but with a Deathrattle (Return a random friendly monster in your hand) can combo with Si 7. Unlock when you finish the rogue class challenge.
You can find many others information on the wiki : http://hearthstone.gamepedia.com/Curse_of_Naxxramas
Best Answer
This is intentional. The bosses in Adventure Mode don't always play by the rules.
Patchwerk starts with no deck but he does not attempt to draw a card at the start of his turn. This means he won't normally take fatigue damage.
However, Patchwerk is not immune to fatigue. If an effect does cause him to draw cards, such as Coldlight Oracle's battlecry, he will attempt to draw them and take fatigue damage as a normal player would.