I would also like to know the best way to tackle this. I believe my husband learned this trait from his mother who shares it. I don't suppose it is what one would really call 'gossip'- more like the need to put other people down and make themselves feel superior. I was raised to give people the benefit of the doubt, and to try to be kind. Evidently not all people were. (no surprise there, I'm sure.)
You go to a home where things are untidy. My first response if someone mentions it is "Maybe they've been sick, or busy, maybe the kids have been a handful lately, etc." They say " What a filthy house! How can they live in that pigsty?" etc.
Another example: You see a very large, obese person. I think " They must have bad health, and they have problems with mobility. I feel sympathy for them." They say " What a lazy pig! If I looked like that I wouldn't show my face in public."
or if someone seems brusque: I think:"Wow, he must be having a bad day. Maybe something really bad is going on behind the scenes, I should pray for him. They say " That guy is such a jerk. I hope he gets whats coming to him!". etc
You get the idea.
It is most often about people they feel they are not so intelligent as they are. They go on and on about whatever "that stupid idiot" did that bugged them.
I find it to be uncharitable, a sign of insecurity, a need to feel superior to another person, and just plain mean.
How do you make it known to your spouse and in-laws that this is not Christian behavior? If I come out and say so, it is bound to bring on bad feelings. It makes me feel bad and kind of angry when I hear them badmouthing everyone all the time. I can imagine what is said behind my back.