I can only speak for myself, but I've had really bad gastric problems on and off, which have coincided with flares of other symptoms and which I'm sure are related to the condition and/or autoimmune thyroid disease. It's varied from extreme, very painful constipation, with blood sometimes mixed in stools (for which I had endoscopy in my mid 20s; they gave up mid-examination as my bowel was too blocked for them to examine very far) to prolonged bouts of severe diarrhoea and weight loss. Also, several episodes of anaemia (low haemoglobin, low ferritin) which suggest either bleeding or poor absorption. I've had blood tests for various kinds of gastric auto-immune conditions, which were all negative. My GP did suggest sending me for another endoscopic examination, but I put it off as the lower abdominal problems haven't been too bad for a while.
More recently, I've had a lot of indigestion-type pain in the upper abdomen, and bad acid reflux which has just started in the last few days, something I've never really had before but which seems to affect quite a lot of people with SjS, from searching old posts on here.