Keep in mind the modern supermarket is a pretty recent thing. We take it for granted that you can get just about anything at any time of year, but it's not something most grandparents would recognise. Heck, even I find there are things in supermarkets that weren't around when I was a kid (Quinoa for instance).
So finding stuff without gluten might have been pretty hard.
Also with anything complex like medicine there are just so many confounding issues that until you have the actual cause, many things sound like they could be it. There would be lots of dead ends and things obscuring the the true cause.
> finding stuff without gluten might have been pretty hard.
Finding anything was pretty hard on its own. In the XIX century, famines due to bad-harvest years were still a thing - potato blight etc etc. It wasn't until early XX century that the combination of industrial practices and a well-established railway network made food scarcity (mostly) a thing of the past, since stuff could now be grown more intensively and then transported quickly all over Europe.
Until then, taking entire foodstuffs out of one's diet could mean the difference between eating and starving.
So finding stuff without gluten might have been pretty hard.
Also with anything complex like medicine there are just so many confounding issues that until you have the actual cause, many things sound like they could be it. There would be lots of dead ends and things obscuring the the true cause.