They are believed to be part of the Tardigrada phylum, a group of extremophile microgranisims that can survive literally any environment, from extreme cold to extreme radiation, these guys are the hardiest of animals.
I FOUND A AMAZING STRATEGY FOR DEFEATING GASBAGS!Get a wyvern and start flying,which will force the gas bags to deflate because they will try to attack you.When they deflate from flying,they have a stunned cooldown to inflate,meaning that they can't attack!
They have a damage resistance when inflated, so use a snow owl to freeze and deflate. After latest update, they can no longer be picked up or hit with bola. Heavily suggested to clear area of hostiles before KO. If timed right, you can keep gasbag in same area while tranqing by alternating freezing and shooting.
A Snow Owl freezing a Gasbags alone will not stop it from re-inflating itself. It'll still take reduced damage. If you freeze it with a snow owl and use the net projectile before it unfreezes it will be ensared while not inflated, allowing you to efficiently and quickly knock it out without chasing it or constantly freezing it.
Shoot it once, then freeze with snow owl. Since you healded it, all is forgiven it wont attack you....so shoot it again.....rinse and repeat. When you freeze it deflates so max dmg/torpor...better then when its trapped and always bloated