There are no listed numbers, but from personal experience and hearing stories from others, I am pretty sure the higher-star the card is, the lower the chances of acquiring it.

The legendary cards from the Gold pack, as you call them, are not only 5-star, but they're 5-star with silver borders, which imo makes them closer to a 6-star rarity. This essentially means that those same Legendary cards are actually rarer than their normal bronze-bordered 5-star counterparts.

tl;dr: The rarer the card, the rarer it is to drop.