Externally regulated gas is comparable to spring, and you still get the nice zero resistance bolt operation, extremely crisp and light trigger, and fully adjustable velocity
I'm running my Tanaka M40 from a 12g CO2 cartridge in a Palmers reg tapped to a long (29rd) mag.
As well as other benefits like an armed indicator (firing pin is visibly cocked), visual inspection of bolt and chamber (can see if you're empty before you close the bolt again), can feed single rounds through the open bolt (good for random tests, other single shot things, or odd shaped/experimental rounds), can decock with a round in the chamber (although you can't recock without double feeding), and it's VERY easy to make the rifle either really loud or really quiet depending on what you do at the muzzle end. Much more than a spring rifle at any rate.
You can also half cycle the bolt and still function (only need to move the bolt far back enough to chamber another round, not all the way back to catch a sear). This in conjunction with the zero resistance bolt operation makes a gas boltie have maybe 3 shots for every 2 from a springer. That's not that important while sniping, but it's there if the SHTF and is always good for a bit of fun.
That said, Tanaka and the KJW clone's hopup system isn't the best, but that's why I have the VSR hopup chamber conversion kit in mine. That plus a firefly rubber and it's shooting quite nicely.
Now I'm seriously wondering, why haven't airsoft sniper rifles gone the way of competition air rifles and moved towards PCP systems yet?
*I'm also biased against springers because it's much harder to cock a springer as a lefty, since you're pulling towards your chest with no hand to control the muzzle rather than to your braced shoulder with your reaction hand out front. Gas is easiest either way.