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I hope the following portion of an article helps answer the question:
How much gluten is safe?

The following was taken from the Sprue-Nik PRESS Eighteenth Edition,
March 1995 "Celiac in the 90s," summarized by Jim Lyles:
Dr. Joseph Murray is now associated with the Mayo Clinic. However at the
time of this article he was associated with the University of Iowa.
Dr. Joseph Murray,  is a gastroenterologist that specializes in treating
Celiac disease. He gave a talk entitled "Celiacs in the 90s" at a
conference hosted by the American Celiac Society on June10-11, 1994.
What follows are highlights of Dr. Murray's talk. Dr. Murray comes from
Ireland, where Celiac Sprue (CS) is much more common. In Ireland, people
have a much easier time dealing with the gluten-free (GF) diet, whereas
in the US it is almost as though it were considered unpatriotic to not
eat wheat.

How gluten-free should the diet be? Dr. Murray believes that Celiacs
should treat gluten the same way they treat rat poison. Celiacs should
never eat food if it is known to contain gluten. Accidental ingestion of
gluten should be avoided as much as possible. For a Celiac, it is
unacceptable for gluten to be ingested more than once a month,
accidentally or otherwise. You can NOT judge whether a food has gluten
by your reaction to it. Many Celiacs can
ingest small amounts of gluten with no symptoms; however, the small
intestine is still being damaged.

Dr. Murray stressed that once you have Celiac disease, you will always
have it; you will never be able to eat wheat or other gluten-containing
products again. This is a fact of life that Celiacs simply must accept
and live with.