Gelsos appear in infinite numbers in some underwater areas of the Kalidus Channel. As soon as one is killed, another one will immediately appear to take its place. They materialize out of thin water at the top of the screen and start descending slowly, pulsating their bodies either to ascend or to aim themselves toward Shanoa. A single hit is usually enough to kill them.
Gelsos are not limited to appear only underwater, though, and they may even spawn above the surface and fly freely in those rooms where they are encountered.
They appear in both rooms where Shanoa must hit exploding barrels attached to underwater balloons to remove debris from the way, in an attempt to make her accidentally hit them before they are in position.