Changing the subject when gossip starts is a small, quiet act of authority.

Gossip continues because everyone present is tacitly agreeing to it, and it stops when one person declines without making a scene. Redirecting the conversation is enough; no lecture is required, and the lecture would only make the group defensive. What people take from it is that this particular room does not run on absent people.

Next time it starts, ask about something else or say honestly that you like the person being discussed. Do it lightly, without the note of correction. Repeat it a few times and people learn not to bring it to you.