In order not to be confused, I will say right away that the pH of gastric juice is normally in the range of 1.5-2.5, this corresponds to a 0.01 N solution of nitric or sulfuric acid.
As far as I know, gastric acidity is not a constant value in the body. It changes over time and from one section of the gastric tract to another. Moreover, the range is from strongly acidic (less than 1) to neutral (6.5-7.5) pH values.
Therefore, the answer to the question depends on what kind of gastric juice to place a person. It is clear that there are open mucous membranes that suffer even from weakly acidic solutions, but if we assume that a person's head is above the surface, then serious problems will begin at a pH below 3. Juice with a pH of 3 and below will cause burns of varying severity. depending on the pH value and the duration of exposure, up to complete destruction of the skin and damage to internal organs. Gastric juice from the body and antrum of the stomach is characterized by such pH values, I don't know where else it is. In the intestine, the outlet of the stomach and the esophagus, such pH should not be, as far as I understand. If you put a person in gastric juice with a pH of 5 or higher, then nothing will happen - this is the pH, for example, of water in a pond with a peat bottom. From 3 to 5 there will be severe irritation and mild burns, I think that the reaction will be quite individual for each person - from almost nothing to serious ailments.
I will answer the question with a question: what happens if a person is thrown into acid? The stomach produces hydrochloric acid (HCl), which, along with other components, participates in the process of decomposition of the incoming food into more elementary components and preparing them for absorption. In practice, if this acid gets on the skin of the body, you will get an extremely unpleasant chemical burn (up to tissue necrosis), and when ingested, burns are formed along the passage of the acid (mucous membranes of the mouth, tongue, esophagus, etc.). Well, if you plump into a container with this acid, then after a while a creature will be found in it, or rather its remains, completely covered with a crust.