Quote from LieutenantSparklezThough it would seem so, I can guarantee that I was not deceived in any way. The Giratina is as normal as any Giratina would be (I've used it in battle many times since I received it), but I get your point. Zygarde certainly isn't half as rare/powerful as compared to Arceus or even Kyurem, so it would seem surprising that I would get such an amazing Pokemon for it.
You have no way of being certain that the Giratina isn't duped. The GameStop events mean that a shiny Giratina is less likely to be duped than, say, a shiny Ho-oh, but it was still duped in all probability.