I don't know if they're the most gossiping people on earth. But the very judgmental nature of the religion with the many man-made rules forbidding or discouraging innocuous things like men having beards and women wearing pants to the meeting creates a large content-set of faults to see in others and gossip about.
Also the level of secrecy surrounding the reasons for someone being announced as no longer being one of Jehovah's Witnesses, naturally arouses curiosity and speculation and, inevitably, gossiping.
So the JW culture is one that is very conducive to gossip. But I don't know if they're the most gossiping.