Like this

uh, I don't really like to say bad things behind people, but I thought there is something you should know. xxx tend to talk about people freely........ The reason I want to bring this up to you is that it affects my work, and I think it also affects other people's moral to see behavior like this in the office not discouraged. I know it is not my place to tell him what he should do, so I think I will just let you know. I won't say a word to anybody else.