Tradition states that Jesus gave an example of how to perform a valid sacrament involves consecrating bread with gluten in them. The Church wants to make sure the sacrament is valid, and because there is no science that can probe whether a sacrament is valid or not, we do not know what sorts of perturbations are allowed.
In any case, the question was why the hosts need to have gluten if the transubstantiation is metaphysical. Just because the change is metaphysical does not mean that the physical conditions required for the change is arbitrary.
The wikipedia you linked state “the change of substance or essence by which the bread and wine…” The word “substance” here refers to the metaphysical substance of Substance Theory, not physical substance. Note that metaphysical does not mean metaphorical.