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.
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