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It isn't often that the discussion in this list turns to geology!  As Bobbi
in Baltimore noted, bentonite is a clay, specifically one that forms from
buried layers of volcanic ash.  It is a mixture of minerals that swell and
expand as they absorb water, and which can also retain certain kinds of
ions well. Many of the "badlands" in the West are there because of
bentonite-rich soils; the clay resembles popcorn when dry and doesn't allow
much vegetation to take hold.

As Bobbi's husband noted, its swelling behavior makes bentonite good to
seal landfills.
I know it's quite harmless in small doses, because the way we geologists
identify a bentonite layer in the field is to take a small piece and chew
it; it's plastic, almost like chewing gum.  Traditional Navajos use small
amounts as a remedy for diarrhea (they call it "hashtl' ish" or "dleesh,"
or white clay).  Putting a lot of it in your gut would probably be
uncomfortable at the least!  It would retain water, but I don't know about
gluten.

The "kao" in Kaopectate is (or was) kaolinite, which is another type of
clay: one that does not appreciably swell when absorbing water.

--Steve in Shiprock