They will attack anything that moves near them, whether it is Tommy or a Hunter. If given a chance they seem to prefer to hunt by lifting their prey up into the sky to be devoured, but when threatened they will retaliate by spitting acid - the same kind of acid that is used in the Acid Sprayer.
They are officially described in game as parasitic creatures that use their highly developed eyesight to track moving targets. Despite this description, holding still does not appear to confuse them. The fact that they are not part of the Sphere forces (seeing as they will attack Hunters on sight) implies that the Gasbags are not creations of the Keepers. It is likely that they were transported onto the sphere during the Harvesting of an alien planet, and have manage to thrive ever since despite the efforts of the Hunters.
Due to their slow moving nature, a lightning charged Leech Gun serves as an excellent way to dispose of them, taking no more than two shots on normal difficulty.
Gasbags have dozens of green glowing eyes at their rear side. They have multiple tentacles at their lower body. Gasbags head resemblance a big mushroom cap.