Gamma rays and X rays are the same thing, just differentiated by energy (and sometimes the method of creation). Under 100 KeV is X ray, over 100 KeV is a Gamma ray. Now, as energy goes up, the gamma ray tends to sail right through the scintillator without interacting, so detection efficiency goes down. You need a bigger scintillator, or a denser one, to have high detection efficiency. On the other end, as energy goes down, the light produced when the low-energy Gamma or X ray is absorbed decreases, so whatever is sensing the light needs to be more sensitive.
(Then, of course, there is Compton scattering of Gamma rays from scintillators, that opens up another whole dimension of complexity.)
Also, the scintillator would need to be shielded from light in most cases, and the shield might absorb the X ray before it even reaches the scintillator.
So, a scintillator intended for low-energy Gamma and X rays would need a very thin window/light shield, while one intended specifically for gammas might use a thicker shield to exclude the lowest energy rays.
Jon