Gastritis has causes.

1) food intolerances-- like gluten

2) infection by viruses (usually short lived) or bacteria (H.pylori)

3) alcohol

4) the dreaded cancer

Gastritis that continues... should be evaluated for causes.

I would also take FLAX oil supplements, or eat it flax containing foods. The essential nature of ALA alpha linolenic acid in food is what builds the protective lining of the stomach.
Fish oils provide long chain fatty acids that are at the END of EFA chemisty. They are anti-inflammatory, but do not do the same things as Flax will. Think of the complex EFA chemistry as a long train. Fish oil is the caboose, and flax is the engine.
There are many cars inbetween! Females convert ALA into EPA and DHA (found in fish oil) up to 25%. That means that 75% of the oil is used by the body for other things, including maintenance and repair.

Axid is an H2 blocker. It blocks histamine and also reduces
acid in the stomach. Acid is needed to break up protein in your food, so it can be digested. When acid is missing because you take drugs to eliminate it, protein is NOT digested and instead moves down to the bacteria in your colon which ferment it and create GAS. All acid blocking drugs do this. It is a serious downside to them. They create a vicious circle of damage over time, while giving a superficial impression of benefit.