When?
How?

What, just rice and potatoes? I can not do it!
How can the success be assessed?
I can well imagine that it is revealing if one suspects to have a histamine intolerance. Because of the reduction to only 2 foods and the daily intake of a new food every 3 days, the reaction of the body can be well estimated. In acute gastritis the stomach can recover in the short time. In chronic gastritis it has absolutely no success, has even rather undesirable side effects, since the diet is restricted already.
*This post is based on research and personal experience. All products (if any mentioned) are bought and tried by myself if not written otherwise.