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I agree with Fran. Very often it is best not to explain, and just say "no
thanks". I only try to explain what I can and cannot eat if the person in
question is likely to offer me food often, e.g. good friends. Most people
don't know what they are eating anyway, so they don't know what they are
trying to feed you either. All the stupid things I've heard... "What,
there's flour in there?!!!" - about pretzels, pasta etc... people seem to
think those grow on trees. If the situation needs an explanation, I usually
go with the allergy, and only if people ask me what would happen if I ate
flour, I say "It destroys my intestinal lining", at which point they
usually decide they don't want to know more about it. Really educating
someone about the gluten free diet is ony worth the trouble if the problem
will come up often.

Sylvia in Ithaca, NY, USA.