The Ginnungagap is a gap between dimensions which can be accessed via the Otherworldly Gate. Within it is the door that separates the human world from the demonic realm, Niflheim. The Summon Spirit Ratatosk serves as the guardian of this door, and his duty is to ensure the door does not open with help from the power of the eight Centurions.
The Otherworldly Gate is a mysterious formation of rocks located in Tethe'alla in Tales of Symphonia. In the past, when Sylvarant and Tethe'alla were separated, it could be used to travel between the two worlds. Now, it is the entrance to the Ginnungagap.