Gold Keys are keys made out of gold.
Background[]
Graham could have received a Gold Key to unlock the door to the Door into Mountain.
Graham found three gold keys, that opened up the the three Magic Doors.
Rosella was given a gold key attached to a rose, that opened up the doors inside of Lolotte's Castle.
Behind the scenes[]
While there are three Gold Keys in KQ2, the inventory item is specifically referred to as a "gold key", singular.