Gastroenteritis. can this last a long time (6+ months so far)? since only small intestinal inflammation has been found through a wide range of tests.

Yes you can: Chronic inflammatory bowel disease like crohn's disease or ulcerative colitis , celiac disease or some other chronic parasitic disease can give you chronic gastroenteritis. Follow up with your gastroenterologist.

Small Intestinal inf: Hi, infectious small intestinal inflammation depending upon the pathogen can last till the infectious organisms has been eradicated. Though this is not that common as most common viral/bacterial causes resolve in 36-48 hrs max 7 days. You should consider other causes particularly crohns, celiacs to mention a few. Please consult a GI esp if you are loosing weight or have diarrhea/abdo pain.