The science on gluten is ongoing and controversial, but the position I believe will ultimately be vindicated is that some people seem to derive no ill effects while many others develop a mild allergy to it, which can fly under the radar and lead to all sorts of chronic inflammation issues if the person keeps eating it. As a result, there's no 'standard safe dosage' of gluten, some people become violently ill if they encounter a trace, others (myself included) find a lot of seemingly unconnected annoyances clear up if they avoid it for a few weeks. Excess weight gain seems to be one of those, though in my case the major symptom was actually an excessively fast metabolism.