The Gelgoogs assigned to the Siren Mobile Fleet received modifications for operation in shoal zones, including sealed joints, additional verniers in the shoulders and knees, and four propellant tanks with thrusters at the end mounted on the backpack. The backpack was also fitted with a beam generator, which provides power to the Gelgoog's beam rifle, its primary weapon, through a cable. It was also armed with a beam naginata and a pair of sub-arms, which were able to hold up to two shields.
Gelgoogs from the Siren Mobile Fleet were dispatched to Side 4 after Fisher Ness reported that the Living Dead Division was having difficulty controlling the shoal zone. They arrive just in time to capture Io Fleming and the destroyed FA-78 Full Armor Gundam (Thunderbolt Ver.).
Daryl Lorenz piloted a Gelgoog during the Battle of A Baoa Qu. However, he had difficulty operating the controls due to his prosthetic arms, and was nearly killed before being saved by an ally piloting a Rick Dom.