Gas timings really aren't indicative of a whole lot in TvZ. Sure, if he gets a lot of gas he will probably build more things that cost gas, but you can't really derive anything else from that. Late gas might also be a quick third or w/e. First off I don't think you really need to scout Zerg until your hellions are out, if you want to feel safe get a marauder and a bunker and you're well off vs almost anyting he can throw at you. Also always make a depot wall. Secondly, if you really want to scout for an all in try to sneak in an SCV at the time the roach warren would go down and then you know for certain. If you control watch towers and leave a marine out front your base you'll have more time to react as well.
gas timings are incredibly important because gas is used to do anything that is unrelated to hatcheries, slowlings, and queens and sunkens. note that none of these things can launch meaningful direct attacks against you at the relevant timings.