In reply to captain paranoia:
> I was surprised to see an advert for gluten-free shampoo the other day.
That's not completely nonsensical -- people who have coeliac disease, as opposed to non-coeliac gluten sensitivity, can react to miniscule traces of gluten, including in skincare (Google tells me some people are fine with gluten in shampoo/body lotion, but some do find they get skin reactions to it). And anything that has, say, wheatgerm in will have gluten.
Of course, I suspect they're advertising it because "gluten-free" is a trendy buzzword and that shampoo never had gluten in it anyway. But for a few people, that's going to be useful information.