They can inflate and then they can do three things, do a huge jump that will deflate them, float/glide, and do a knockback attack that is similar to the diplodocus' only attack. They can also seat more than one person at a time
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
If pure tanking power is tanking into account, this thing is second to none. Just like Rock Elementals, the Gasbags has a 80% damage reduction from almost everything (as long as it is at least partly inflated). The Gasbags, however, is much easier to tame, can be bred and has a saddle which is not that hard to find at all. Their smaller hitbox is welcome when using Velonasaurs and guns from behind them as well.
The only downside is their low damage, and also that they can't damage stone (and turrets), so don't expect to raid entire bases with it alone.
When damaged, a bit of gas will escape, so unmounted ones might find themselves taking full damage after a while. If mounting is not a option, be sure to put it on Neutral from time to time so it will inflate by itself as part of the attack behavior.