<<Disclaimer:  Verify this information before applying it to your situation.>>

I am not an expert but I can assure you that Horsetail is absolutely
gluten free were it not for the 100 million years or so which separate
Equisetaceae (i.e. the family to which horsetail (Equisetum arvense L. or
kin species) belongs) from Gramineae (i.e. the family of wheat, rice, corn etc.)
where gluten is found.
It's very interesting what you say about silica: it's absolutely true
that horsetail is very rich of these substances and I agree on the fact
which they can effectively protect the gut mucose. The only thing you should
be careful is that drinking too much horsetail can have some unpleasant
effect on kidneys (it's in fact a bit irritating). You can take it for a certain
period (say two months) and stop for a while (say one month) and so on.
Let me know if you notice beneficial effect in case of celiac disease: I am
interested because my son is autistic and can't tolerate gluten and casein.
Pierluigi Fortini