A gold key is nothing more than a key at first glance. However it turns out that it is made of solid gold, and can be useful for getting some more valuable riches.
Obtaining a gold key can be either tricky or easy depending on how much effort you have put into the game thus far. To get a gold key you must kill a monster in a dungeon. They will sometimes drop a key for you. Be careful though as they can be strong.
Gold keys are the keys to the riches of ancient times. These gold keys unlock special chests in dungeons which tend to have much better rewards than ordinary chests.