Gastritis is inflammation of the stomach. Gastric atrophy, or more accurately atrophic gastritis, is the same thing, when it has been going on over a long period of time. It can eventually lead to pernicious anaemia. If you have pernicious anaemia, you'll need regular injections of B12 as it prevents B12 absorption. If you don't have pernicious anaemia but avoid meat, you can just take a supplement. Methylcobalamin is the most bioavailable form.