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 the player. A single hit is usually enough to kill them.
Gelsos are not limited to appear underwater and may even spawn above the surface and fly freely in those rooms where they are encountered.
They appear in both rooms with exploding barrels attached to underwater balloons used to remove debris from the way, in an attempt to make the player hit them by accident before they are in position.