Why are the contents of gastric juice so hostile to the exposed duodenal wall?

Gastric juice  is a thin strongly acidic (pH 1 - 3 ) almost colorless liquid secreted by the glands in the stomach lining. It is made acidic by one of its constituents namely hydrochloric acid. It is because of its acidic nature that gastric juice can be  so corrosive to the lining of the duodenum.

The integrity of the inner lining of the (mucosa) of the duodenum depends on a careful balance of the "aggressive" factors (such as acid) and "defensive" factors (such as mucus) which helps to protect the delicate surface lining.

Disruption of this balance caused by too much acid (or weakened defense ) can result in erosions or ulcers .

answer; eat a strict gluten-free diet;