The Great Pit of Giga is the sealed entrance to the dark world outside of the Castle of Baramos. After the defeat of Baramos, Zoma breaks the seal on the Great Pit of Giga allowing the hero and this party to enter Alefgard, where the rest of the quest continues.
Notes
There is also a sword called the Sword of Giga, that has nothing to do with the pit it's self.