Do not say this. It is like you think this precursor makes 'okay' whatever you are about to tell us about someone. You KNOW you are gossiping, but here it comes anyway. Just.stop.your.self! At some point, your inner-monologue must think, "oh no, words are coming out!!!"
Other variations: "I'm not trying to be a gossip, but..." "I hate to be a gossip, but..." "I told her I would not tell anyone else, but..." "Is it gossip if I tell you that...?" "I think we need to pray for..."
Usually followed by: "Oh, she would probably tell you the same thing." "This is just between you and me."