the "THEORY" of the 3rd gear looks believable, but i do wonder if it would actually work properly and if it would be as lightweight to allow great mobility, also the gas canister looks rather small and im not sure if it would fit enough gas to even lift a person in the air.
There, of course would be the issue of, when in a modern day city, all the bigger buildings with glass for walls instead of stone, concrete not being as sturdy as actual stone for your 'catch' end of your harpoon to latch onto and hold. Then there's the problem that most forests on earth don't have huge trees in them, and so aren't spread out enough that you'd smack into every other tree as you flew through the air. (Let's not forget that in the show their harpoons seem to let go automatically for the next launch. If you could get one to stick in something at all you'd then need a way to reel it back in without leaving the harpoon snapped off the end of your cord/chains). The best place to use something like that would be a canyon(more specifically, the grand canyon, but there are of course, others), but if you needed to make a sudden turn and you did not have sight of a place to swing yourself around, you'd smack into a wall. Even if you went to an older city where most everything was made of stone or brick still, there's still be telephone poles and their wires to deal with (and fire escapes. One tangled harpoon cord...), and most of those houses would not be very large. If you missed your latch-on spot, you'd have next to no time to get off a second shot, and you'd fall either to the ground (if you're lucky) onto a person, a trash can, or a car, or, in the worse case scenarios, either get impaled on a fence or land in the middle of traffic. But I do agree that it'd be cool to actually be able to use it.