So I solved the problem, I think. The others steered the conversation in another direction. And when the topic did come up I found something to praise about the

So I solved the problem, I think. The others steered the conversation in another direction. And when the topic did come up I found something to praise about the victim of the gossip (a small thing, since we don't work together), but I deliberately and obviously exaggerated its importance, to signal that I didn't like the one-sided nature of things. That, and the fact that I pointedly ignored him when he started gossiping, seems to have worked. Thanks to all who responded.