I don't see where GC have a short gut compared to other fish.
From Dr. Griffin's talk at Arlington, pretty much all fish have short guts compared to mammals. CC, with a gut length:body length ratio of 2:1, have the longest gut:body ratio of any fish he cited. (By comparison, the big cats have some of the shortest ratios among mammals, at about 4:1).
In general,the gut:body ratio is higher for low protein/high plant matter diets. Given that GC eat almost solely on plant matter, I would expect their gut:body ratio to be higher than that of catfish.