Answer:
I believe a 'fellow' is a student that already has earned his/her MD and is studying (in this case gastrenterology) under a senior MD (again, in this case a gastroenterologist), usually at a large, teaching hospital. The selection of fellows is usually at the senior doctor's descretion, and the senior is responsible for overseeing his fellows and they in turn consult with him on all cases. It is a great way for young doctors to learn under a knowledgable specialist and encounter rare and difficult cases unique to large, urban hospitals.