If I recall correctly, a lot of the gastric stuff is due to chitin in the cell walls, which humans generally can't digest. Probably also related to the fact that most people eat them dry. I could be entirely repeating bro science, idk.

People have reported making tea with the mushrooms (unlike THC, psilocybin is water-soluble), or soaking them in lemon juice (idea, I think, is that the acidity breaks up the cell walls a bit more) prior to ingestion helps. Don't trust me, though, look into these independently if it's of interest.