It is the combined form of the three Experimental Animal Yo-Kai.
Gamegosu is a giant, purple, three-headed, turtle-like Yo-kai with darker purple skin on its long tail and each of its heads.
On its back it has a dark purple shell with five pale red-spotted spikes on it, three large ones and two smaller ones. Its arms and legs have a wavy zigzagged pattern on them and its hands and feet are clawed.
Each of its three heads has horns, turquoise tounges, red irises, and two large fangs respectively in addition to multiple smaller fangs. The number of horns on the heads varies, along with some other features. The rightmost head has two horns, green scleras, more angular eyelids, and a much longer tounge that the other two. The middle head has one horn, blue scleras and a more jagged muzzle. The leftmost head has three horns, orange scleras, and eyelids that curve upwards instead of downwards.
Its underbelly is a light cream color and there appears to be a red diamond-shaped mark with a black spiral on the center of the chest.
Gamegosu is shown to be very aggressive and irritable.
Gamegosu is shown to be an excellent swimmer, moving through the water at high speeds, jumping in and out of the water rather quickly. Futhermore, it can slash at the enemy with its sharp claws, and also ram the enemy at full force. It can also shoot out water balls from its mouths at a very high pressure, called "Gamego Water Bombs".
At some point after Dr. Maddiman released the Experimental Animal Yo-kai into the sea, they were fused into Gamegosu through a mysterious power.
Seeing the giant beast about to reach the bay, Dr. Maddiman calls over Inaho and USApyon for help, but they are at loss about what to do, too. When Sergeant Burly and Meganyan appear on the scene, Inaho has the idea of using Meganyan's powers to turn the plastic robot model they had made for Eddie earlier into a giant version of itself and use that to battle Gamegosu.
Inaho and USApyon board the giant robot Dainamerashin and defeat Gamegosu with it, who then splits back up into the three turtle Yo-kai.
The name "Gamegosu" is derived from kame (Japanese: 亀 "turtle"), and is done in a style similar to typical kaiju names.
Gamegosu is on of the Yo-kai that is based on Kaiju, another being Dozilla.