Gasbags are an enemy in The Consuming Shadow. They are one of the new enemy variants that appear in the Insanity Edition.
Any common enemy can randomly be converted into a Gasbag type, meaning they can't be hit at close range without suffering damage. Like a dying Morass, try to keep your distance when fighting them.
In-game description Edit
Whatever blasted hellscape the creatures of the shadow came from, they may not have entirely left it behind. Various minions have been observed to constantly emit clouds of toxic gas that doesn't seem to have any biological source within their body. Best guess, whatever magic teleported them here left a few parts of them behind in a homeworld with a high-pressure toxic atmosphere. While it is never wise to get too close to a shadow minion, that goes doubly true for creatures with the affliction.