According to the story, as revealed by Fina when she joins the party, the Gigas were created as weapons of mass destruction by each of the six civilizations of the Old World. Each Gigas has a different form and attacks, possessing great amounts of power and are controlled through use of a crystal corresponding to its color.
Aside from the Purple Gigas, they are fought as ship boss battles throughout the game. Two of them (Recumen and Grendel) are Little Jack's battles (though it is impossible to win the battle against Recumen); the other three (Bluheim, Yeligar and Zelos) are taken down by the Delphinus. The Purple Gigas, Plergoth, also known as Rhaknam, was sought after by Drachma for the death of his son, but ultimately was killed by Ramirez's ship.
Fina states that Gigas are incapable of making their own decisions and follow the will of the one who awakens them. What becomes of the Gigas if their master is incapacitated seems to vary from one to the other. In most cases, the Gigas will cease functioning. However, Grendel goes on an uncontrollable rampage when the Ixa'Takan king is knocked unconscious - a similar case occurred with Rhaknam, who wandered the skies for centuries, destroying ships seemingly unknowingly, after his master perished.
The journey through the Dark Rift hints heavily at a lost Black Moon, possibly because of the Valua-Nasr War and the Rains of Destruction. If there is a Gigas for each Moon, there could possibly be a Black Gigas - however this is only theoretical.
Each Gigas represents the element of given moon.
- The strongest Gigas is Zelos, followed by Yeligar, as it was so strong that the Silvite Mages couldn't separate it from its crystal.
- Rhaknam and Yeligar are the only two Gigas who remained in possession with their respective Moon Crystals.
- Recumen and Grendel are the only Gigas still alive at the end of the game.
- Rhaknam is the only Gigas known to have been created from a living creature.
- Every Gigas name, with the exception of Zelos, begins with the letter that spells the color of the moon that it represents: Recumen = Red, Grendel = Green, Bluheim = Blue, Plergoth = Purple, Yeligar = Yellow. Zelos is the only Gigas that doesn’t follow this pattern, although the name of its location does, Soltis = Silver.