I actually dislike almost every answer thus far, as they all get at pieces of what's going on, but even together too much is missing.
Let's start with the cost of a gas steal. If we assume that this is a scouting worker that would not have been mining anyway then the costs are as follows:
- Protoss -> 75 Minerals
- Terran -> 75 Minerals + 30 Build time (which if your worker would have survived is 30 minerals)
- Zerg -> 75 Minerals + 20 Build time for new drone (which if your worker would have survived is 20 minerals)
So we can see that for Terran this is actually a more costly strategy. What's more if he wants to monopolize your gas you're looking at even higher costs for Terran and Zerg.
Now if a gas steal is so expensive, why are people doing it? There are a couple reasons:
- Prevent early cheese (Banshee, Reaper, Void Ray)
- Force you down a certain build
- Scouting purposes
For Zerg that third one is most common, and if you're not cheesing I suppose you don't have to worry about the first, so let's focus on that second one, as its the really dangerous one. If you're playing Terran (for example) and a Zerg steals both your gas, its likely you're going to see some quick Roach or Baneling play. So take this into account.
The most reasonable response to a Gas steal, is to take your other Gas. In a worst case scenario, you've wasted as much on Gas extraction as he has. Now don't go overboard and start building gas at 13 on every game, only when he takes one of your gas.
So what if he takes both? In your example the Terran player spent 210 Minerals on gaining a Gas monopoly. As a result he can't easily mount a significant offensive. In this situation, Chrono boosting out a couple Zealots to clear your gas, while using your excess minerals to get a Cybernetics Core and possibly move into 3 Gate is a perfectly reasonable response.
He's seriously handicapped himself; your strategy is largely unaffected, and you'll be in good position once your gas starts up. In this scenario he's really at a disadvantage.
Now you might consider, what happens if he sends a Reaper? Early Reaper play requires a significant economic sacrifice, as does a Gas steal, so the presence of one will necessarily delay the other. If he's stealing both your gas, he doesn't have the economy for a super fast Reaper. Now he can still build one, but it probably won't arrive till you have your Core and a couple Zealots up (and probably the Refineries down).
What people do need to watch for, in a gas steal, is something like Protoss stealing both gas vs Terran. This will naturally force Terran into a more Marine centric build which the Protoss can exploit with a Zealot heavy Immortal timing push. In that scenario I would suggest throwing down extra Barracks (without add ons obviously) and pumping out Marines to clear out the gas. These extra Barracks can then get Techlabs, and you should be able to field a decent sized Marauder/Marine force by the time he arrives.