Cranberry. Regular consumption of cranberry juice, could prevent infections with Helicobacter pylori bacteria^1,2, in particular by preventing the bacteria from adhering to the wall of the stomach³. Taken with a conventional treatment, it would also allow the bacteria to be neutralized more quickly.⁴. 

 probiotics. Study suggested probiotics would have cranberry-like effects⁵.

 Apricots. Japanese apricots, 3 per day, may reduce inflammatory effects of H. pylori bacteria in chronic gastritis, study found⁶. 

 Linen. Flax seeds may relieve irritation of the mucous membranes caused by gastritis.