Is there a medication to treat thickened stomach wall due to chronic gastritis? or treating underlying chronic gastritis is the only way to handle it?

Not really: treating the gastritis might be a way to treat it

Treat the gastritis: The concern with gastric wall thickening is that it can be hard to distinguish from malignancy. There is no real problem with having a thick wall.......instead, the problems arise from the symptoms of the gastritis. So, the single best thing to do is see your doctor. Get endoscopy with biopsies if needed. And get adequate treatment for the gastritis....and the symptoms you are having.

Yes: That is correct, you treat the underlying issue which is the gastritis.

Gastroenterologist: Has a gastroenterologist reviewed your findings and felt that a gastroscopy isn't advisable and that there is no concern about malignancy? Has h pylori been ruled out? If so then chronic gastritis is treated with avoidance of alcohol,caffeine,nicotine, spicy foods and the taking of medications like prilosec or zantac. I feel that you should be followed by a gastroenterologist.