Its a very common misunderstanding, but also true for many practical applications. X-rays used to take pictures of the skeleton are generally much lower in energy than the photons produced in nuclear reactions.
There is, however, quite some overlap: The most energetic x-rays have more enery per photon than the weakest ones produced in nuclear reactions.
Back in the day when these terms were coined, there was very little overlap, and a distiction on energy could be made. The terminology changed to indicate the origin, as both are exactly the same EM radiation, with the exact same properties etc. Mistaking one for the other is of little consequences, and even many textbooks still cite the distinction in energy, though usually without a precise point where x-rays stop and gamma rays begin.
Obviously the same problem goes for the distinction between UV light and x-rays, but an arbitrary boundry as been defined at 10 nm to indicate the difference.