Gastritis is a broad term and should be further qualified. There are several causes for it such as H.pylori infection, ingestion of pain killer medicines, etc. Once the cause is identified, the appropriate treatment should be taken. Endoscopy is necessary to diagnose it, which you would have undergone.
Nexium is an acid suppressant drug and is safe to be taken for symptomatic relief. I note that you are overweight, and regular walks would be good for you. Exercise would be good for you, and weight reduction also helps in reducing reflux related symptoms.