You have a medical condition that affects your digestive system. You may have symptoms such as diarrhea, constipation, gas, or pain. Or you might not feel well at all. Or maybe you do not know what is happening with your digestive system and you just want to find out more about what a gastroenterologist does. A gastroenterologist is a doctor who specializes in the diagnosis and treatment of diseases affecting the stomach, intestines, or both. They are also called an endoscopist or an intestine doctor. You probably already know that if something is wrong with your stomach it might cause stomach aches and/or nausea.