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.
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.
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.