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
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.