You shouldn't reveal information about someone else without them knowing, unless it is something about them that they have already made known. I believe if someone tells me something, and hasn't stated anything in the conversation letting me know they have told others, I keep it to myself.
It's the 'telephone game' effect, and that's usually how it becomes gossip. I'm told something, I turn around and whisper it in someone's ear, I might not state it correctly, or they might not have heard it right. Then, they go and tell someone else, and before you know it, everyone and their mother has heard it, and possibly not even the correct story. This is when gossip can become hurtful or damaging as well.
To sum it up. Don't reveal things about others unless you know that you have permission to do so.