Why does the need for gluten-free/reduced bread matter if the bread is no longer bread?
Sacraments require proper matter in order to be valid. You cannot baptize someone by pouring glue over them, you must use water. Using grape jelly instead of oil is not a valid administration of the Last Rites. In the same way, bread used for the Eucharist must truly be bread. Even though the substance will be changed through transubstantiation valid matter is still required for the sacrament to happen.