Beets and activated charcoal will let you know what your bowel transit time is. Obviously, beets will produce a red color in your stool (don't forget you ate them, or it could be scary, and the charcoal will turn your bm black.
Activated charcoal is used commonly to determine GI transit time. It is available in most drug/supplement sections.
Only the way it absorbs toxins which could help reduce the impact of gluten. Though some nutrients would be absorbed to but I doubt there was much nutrition in that gluten meal. A quick search for celiac and "activated charcoal" shows many results.