The story of Gagagigo is a lot more elaborate than that.
His story started out in his younger days, where he served Eria the Water Charmer as her familiar. During this time, he fought alongside her and many others. Through the use of an unknown power he grew up into Gagagigo, and continued to serve Eria for a long time before leaving her though he became a lot more violent. This influenced Eria's behavior slightly as she later became Raging Eria. He went on to follow his own path of evil where he ran into Freed the Brave Wanderer. They both started to fight and Freed used his ability to banish Gagagigo into Dark World, the land of Demons. Gagagigo was captured by the Archfiend army and was going to be used as a sacrifice when he was rescued by the valiant Marauding Captain. This caused Gagagigo's heart to change. He joined up with the Marauding Captain for a short time to help him in battle and one day he sacrificed himself to protect one of his men from the Invader of Darkness. in the same way the Marauding Captain had done for him years before. After this defeat Gagagigo realized that he needed more power so he turned to the twisted Dr. Kozaky for improvements. He went on to gain aid from Kozaky, and got his body reconstructed, turning him into Giga Gagagigo. This was a mistake and Giga Gagagigo was turned against his friends by Kozaky's Dark World technology. Soon, Giga Gagagigo was sent back to his dimension by a Monster Gate and he became a fighting machine and sought more power. He attacked Freed the Brave Wanderer once again and it is implied that he was the winner of that battle. Due to his cybernetic implants, he became power-hungry and transformed into Gogiga Gagagigo.