Not to worry - users who have been around far longer than you have made the same mistake. The region restrictions aren't obvious unless you go looking for them. I myself won a GA from this bundle that I couldn't activate because of a region lock, which is why I thought I'd share what I discovered.
I don't think even Support can add region restrictions to a GA after it has begun because people would have already entered. If the winner can't activate it (you may want to ask about region before sending the key), you will have to hope that they agree to delete the GA, after which you can open a support ticket for it. If they don't agree, you may end up with a "Not Received" feedback but it's not the end of the world.