I decided to look up the meaning of the word "fad"

an intense and widely shared enthusiasm for something, especially one that is short-lived and without basis in the object's qualities; a craze.

So calling going gluten free a fad has the effect of minimizing its importance. Lots of people consider low carb or keto to be fads. Calling something a fad is a way of demeaning it. For many people, myself included, going gluten free is necessary if they are to maintain their health. I would prefer that the word "fad" not be used to describe gluten free eating just as I don't like seeing it used to describe low carb eating.