“What can you eat when you have gastritis and diarrhea?”

Gastritis is by definition an inflammation of the gastric lining which could be caused by multiple causes. Most cases are treated empirically with antacids and dietary restrictions like the avoidance of spices, alcohol, etc.
If not improvement, patient should be further evaluated with an endoscopy.
Diarrhea can have multiple causes however the supportive treatment is common to all and include hydration, electrolyte replacement, avoidance of dairies and sugars.
If diarrhea persist for over 72 hours, has mucus, blood or patient starts experiencing fever and chills further evaluation by a physician should be done.

Hi! Sorry to hear you are suffering. Often with Gastritis and Diarrhea, you may be dehydrated if these symptoms are prolonged, so I definitely recommend speaking with your Primary Care Physician (any Internal Medicine or General Practice Physician will do) as if these symptoms last longer than a week or you start to feel dizzy or are completely unable to eat or start vomiting along with these symptoms or are getting worse. The recommended diet is called a "BRAT diet", and generally consists of Bananas, Rice, Applesauce and Toast. This diet is helpful for anyone who has nausea, vomiting or diarrhea. Hope this helps answer your question!

The best food to eat is a very bland diet (sometimes called a BRAT diet) consisting of bread, crackers, and plenty of water. Avoid large meals, spicy foods, greasy/fried foods and caffeine

You can eat whatever you tolerate. Having gastritis does not usually limit what can be eaten. With regards to diarrhea, we need to know the cause of the diarrhea before advising on diet. There are so many potential explanations for diarrhea. You need a diagnosis first, then can dietary recommendations can be made. If the diarrhea is from IBS, then the low FODMAP diet can help. If diarrhea is from celiac disease, then a gluten-free diet is needed. If diarrhea is from infection, then diet won't make any difference. Etc.