The Gold is in box B.

Lets assume that label A is true. This means that label B is also true and the gold is in box A. In this case, the label A is false. This CAN’T be, because we assumed that the label A is true. Thus, our first assumption is invalid. So now we know that label A is lying. This leads us to two cases: (1) The label B is false. (2) The gold is in box B (now that we know that it cant be in box A). If (1) was true, then the gold is in box B or label A is true – which can’t be because we know that label A is lying. So in BOTH CASES the gold IS IN BOX B.