Location: Island Cave
Once you have the three Regis, Regirock, Regice and Registeel, you can unlock Regigigas. The method to get this is rather complicated. In Pacifidlog Town, in the north-eastern house, a girl will tell you stories her grandfather told her. At random, she will tell you a part of the story about how to get Regigigas. This involves location and the other features. Take all three of the Pokémon to the Island Cave during the day, and make Regice hold a frozen item such as CasteliaCone, Snowball or Never-Melt Ice, and have Regice have a nickname, and Regigigas will challenge you.