Seitan is made by the simple process of washing the starch out of wheat dough, leaving the gluten. It should be remembered that seitan, although this name for it is new, is a traditional food in countries such as Japan and China, where it has been eaten for more than a thousand years (wikipedia says its use is documented at least as far back as the 6th century). Of course, just because something is traditional and prepared in a low-tech way, does not mean it is necessarily healthy, but it is interesting that coeliac disease seems to be rare in areas where seitan is a traditional food. It would seem that eating seitan does not necessarily cause people to develop coeliac disease and other gluten sensitivities.
The increased prevalence of coeliac disease is a puzzling phenomenon and there seems to be quite a lot of research into it. The protein in gluten that seems to be mainly responsible for coeliac disease is gliadin.
Different wheat varieties differ in the amount of gluten and of gliadin they contain. The modern wheat varieties grown in countries where coeliac disease is relatively common (Europe and the USA), also called dwarf wheat, bred over centuries for high yield, seems to be generally higher in the more inflammatory types of gliadin associated with coeliac disease.
Another interesting factor is that sulphur and nitrogen fertilizer change the proportion of different proteins formed in wheat. This article notes that the fourfold-increase in coeliac disease since the mid twentieth century has occurred since the use of chemical fertilisers became widespread in Europe, the US and other "industrialised" countries, and that those are the countries where coeliac disease is most prevalent. If I find research into the protein composition of organic versus conventional wheat, I will add them here.
Gluten-free seems to be one of the most confused and confusing trends in modern food culture. I am frequently asked if, as a vegan, I can eat bread, since it contains gluten, and I am frequently told, when I ask if a product is vegan "yes, it's completely gluten-free". I have read several confusing articles saying that coeliac disease is on the rise because of all the starch we eat, "our ancestors didn't eat all these refined starches" etc, although coeliac disease is caused by proteins. I very much hope that research will conclusively explain what is causing gluten sensitivity to increase and thus how we can reverse this rise.
In the meantime, my conclusion is that seitan itself is not what should be avoided (based on its long history of apparently safe consumption); if anything, it's particular types of wheat eaten in excess. It may be better to choose organic wheat, which may have lower levels of inflammatory gliadins due to non-application of fertilisers, or to choose "heritage" varieties like spelt which seem to be less inflammatory. Further research is needed, but, it seems, seitan is not the enemy here.
Of course, if you are a coeliac or have another type of gluten intolerance, you must avoid seitan along with all foods containing gluten