Considering there's whole shelves of grocery stores dedicated to gluten free and how big that's gotten I think it's further towards fad and marketing vs. scientifically supported condition. Lots of confounding factors (i.e. carbs in general, or things in wheat/grains other than gluten). Plus placebo, plus psychosomatic (it's got gluten, I'm allergic, so I must feel sick).
Makes for lots of variety for those with Celiac Disease tho
All the heath professionals I've spoken with (including an allergist) don't put much stock into it and say that a good gluten-free diet takes careful attention since it's easy to have too much fat and not enough fibre.