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You're right, but it's not a subversion of the Gambler's Fallacy, it's a subversion of conditional probability. A classic example is that I have two kids, and at least one of them is a boy. What is the probability that I have two boys?

The intuitive answer is 50%, because one kid's sex doesn't affect the other. But when I told you that I have two kids, there were four possibilities: GG, GB, BG, or BB. When I told you that at least one of them is a boy, all I did was take away the GG option. That means there's only a 1 in 3 chance that I have two boys.

But by having one child answer the door, I change it yet again–now we know the sex of a particular child. We know that the child who opened the door is a boy. This is now akin to saying "I have two children, and the eldest is a boy. What is the possibility that I have two boys?" It's a sneaky nerd snipe, because it targets specifically people who know enough about statistics to know what conditional probability is. It's also a dangerous nerd snipe, because it's entirely possible that my reasoning is wrong!