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.
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.
How I knocked out a high level one in single player is I used my looked around the waste land with my poison wyvern level 120 (bean cosumer) . Found a highnlevel one grabbed it with the wyvern (press c on pc while flying)took it home. I put it in this small stone house and traped it in there got my level 225 equius in there and knocked it out after a while. If its trapped you can use any level of torpor inflicting creature as long as it won't kill it . By far the fastest way to knock is a high Melee basilisk which you don't even need to trap the gasbags with. But I don't most people have one of those as they take fertilized rock drake eggs to tame and I am still having trouble stealing one of those.
I had the server 1st Gasbag tame on legacy 915, lvl 145M. You can bola it, but it can still try and float away from you but when it lands it's still trapped. Headshots and lots of tranq arrows. Took about 130 to knock out. I kited mine into the city and under a bridge. Torpor drops super slow, used mejos and took about 11hrs. Its base weight was 4,400. It tamed out at lvl 150. Lvl oxygen so you can float longer. THEY DO TAKE FALL DAMAGE. To operate once tamed Rclick to inhale. Lclick to peck attack. C for a really fun knock back that works on wild dinos, knocked out one and players even on pve. A great trolling mechanic. Be gentle with flying it when you first get it until you learn how it handles. Inhale then space gives you a nice jump but do not hold W key. It will take you super high... then no air and then a nasty belly flop and you may kill your tame. Just give little direction corrections with the keys and you'd get a feel for it. I love this thing to death. Cameron-Forsaken 915
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