For a number of years, our priest used gluten-free bread. If I received, was the host truly consecrated?
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.
In 2003, the Congregation for the Doctrine of the Faith ruled that hosts must contain at least some gluten to be valid:
1. Hosts that are completely gluten-free are invalid matter for the celebration of the Eucharist.
2. Low-gluten hosts (partially gluten-free) are valid matter, provided they contain a sufficient amount of gluten to obtain the confection of bread without the addition of foreign materials and without the use of procedures that would alter the nature of bread.
Hosts used for Mass must contain gluten to be valid matter. However there are low-gluten hosts that contain such small amounts of gluten (as little as .01%) that many persons with celiac disease are able to receive them. My presumption would be that the priest used a low-gluten host.