Marion Nestle summarises the status of gluten-free hosts and usefully links to the Vatican's circular on the matter. I actually went to look, wondering whether, maybe, there's some reason why the host cannot be gluten free. And there is, up to a point.

The bread used in the celebration of the Most Holy Eucharistic Sacrifice must be unleavened, purely of wheat ...

But that's not really a reason, is it. It's just a historical tradition. Obey, or else.