Speaking as an educated lay person, it seems unlikely gut problems are behind everything. It also seems unlikely that the gut is not involved in systemic health at all.

A search here for gut will get you a lot of results. Some are articles in the popular press, but others are from journals, such as this one from July 2022 in Nature:

Diet and its effects on the gut biome in the pathophysiology of mental disorders (nature.com) https://news.ycombinator.com/item?id=32221449

There’s good discussion on this site, from medical professionals and educated lay people.