Gossip is "idle talk or rumor, especially about the personal or private affairs of others" (definition found at dictionary.reference.com). It's not something you should feel compelled to share with others. If you "heard" something about someone, not actually saw that rumor in action, it's probably not a good idea to be spreading it around. Especially if you work with the person or see them regularly enough that it could come back to you. Nobody likes a gossip, and it's worse if you're gossiping about things that just aren't true. Gossip is not honesty. It's talking about something that is most likely none of your business without knowing all the facts of the matter. If you don't have all the facts, you can't really be honest.
Disclosure is quite a different animal from gossip. Disclosure seems to fall more into the category of giving someone information you feel you are legally or ethically bound to give them. Your doctor will tell you that shortness of breath is a side effect of this or that drug. That's disclosure. The building is being exterminated for rats. That, too, is disclosure. But what about a situation like this: Person A says that Person Y was accused of something like drinking on the job. Accused is not the same as convicted; it's not even the same as having a complaint filed. What if you know Y? What if Y has never been anything but helpful and professional to you? When Person A tells you this about Y, it seems not only like unnecessary information--because it seems like you might have noticed that kind of behavior earlier, but it seems like information that may be false or perhaps hyperbolized to the point that it is a misrepresentation of the facts. While it may seem like a person is ethically bound to disclose that kind of information, it's important to do it because one is concerned for another's welfare, and not because the person may have jumped to conclusions, or is telling stories about someone just because the person doesn't like them.
This one doesn't seem to be as relevant to honesty as the others did; however, if we examine it a little more closely, it is relevant in certain contexts. When we are being honest with someone--I like your new dress, I don't like that restaurant, I'd rather sit farther away from those noisy kids, etc--we are in fact passing judgment. We like the dress--it looks nice on her. That restaurant is perhaps dirty, crowded, unfriendly, has bad food, whatever. I don't like kids; the kids are noisy because their parents have no idea what they're doing; kids shouldn't be in restaurants because they can't be kept quiet, and so on. Those are all judgments, some stronger than others, but judgments none the less. Every preference you give is a reaction to something else--that reaction, although it may not be strong or visceral--is a judgment in favor of or in disapproval of something else. Judgment is something that can't be helped. But when you're being honest with someone and sharing your judgment about a situation, person, outfit, or whatever it may be, some judgments are better kept to yourself. Example: Saying "I hate that city," to someone from that place is not a very nice or tactful judgment to share with them. So, while sometimes honesty really is the best policy, honesty is not the best policy all the time.
I apologize for being a bit preachy today, and I realize that the disclosure example was a bit extreme with A and Y, but I have a point. We need to be aware of what we're saying, why we're saying it, and how it's affecting the people to whom we say it. This kind of awareness is valuable. It helps keep one's foot out of one's mouth more of the time.
Honesty is not equivalent to tact. Tactfulness helps.