My stool is green. Is it gastritis?

Not necessarily. All stool starts out green in the small bowel. If it goes through the large bowel too quickly to oxidize, it can stay green.
Maybe. You dot not say if your stools are loose or formed, and how long this has gone on. You also don't mention number of stools a day. If you are having a few days of loose and frequent stools that are greenish, it can be due to to an to an intestinal virus. It can also be due to diet, what you eat/drink. Gastritis applies only to an inflammation of the stomach and is usually accompanied by pain.