From Sonic Retro
The Gaia Colossus is the entity that Chip creates in Sonic Unleashed using his Light Gaia power to bring together the Gaia Temples with the 7 Chaos Emeralds. Together the Temples interlock to form the Gaia Colossus.
In the fight against Dark Gaia, the 1st stage is to get to Dark Gaia using the Colossus. You need to block and dodge oncoming rocks. You MUST block the laser. Once you reach him, you must hit the QTE's as shown to reach stage 2 of the fight. You must repeat this 2 more times. Take too much damage or miss too many QTE's, and you will lose a life.
In the 3rd Stage of the fight, the Gaia Colossus fights Perfect Dark Gaia in his shield while Super Sonic must disable the shield to finish him off. If Super Sonic takes too long, the Gaia Colossus will lose all its remaining health fending off Perfect Dark Gaia and you will lose a life.
The 1st Stage involves fighting Gaia ala Punch-Out, dodging and hitting him.
The 3rd Stage has the Gaia Colossus absent in this version.
After the Fight
Once you win, Sonic loses his super ability, and is saved from falling into the lava by the Gaia Colossus. Chip thanks Sonic for all he has done, before promptly throwing Sonic out of the planet's core.
This being seems to have been based off many shows where a team's beings combine into a larger one, like the Power Rangers.