The danger is that gossip can lead to the sins of rash judgment, detraction, and calumny (slander). Rash judgment occurs when a person accepts something as true without real evidence. If a person says, "Did you hear what happened?" listeners should ask themselves how the speaker knows this. To repeat what someone else has said is not evidence; in court it is hearsay. Did the speaker witness the event and interpret it correctly? It is sinful to believe someone has done wrong when there is little or no real evidence. Even when you know something has occurred, it is open to several interpretations. In politics they call it "spin". The church invites us to put a favorable interpretation on the event whenever possible. CCC sec. 2478. To repeat a true story about another's wrongdoing, to someone who has no need to know it, is the sin of detraction. A person is guilty "who, without objectively valid reason, discloses another's faults and failings to persons who did not know them " CCC 2477.
"'Backbiting' -- that is, talking evil of persons behind their backs. You would not like your neighbor to backbite you, and you have no right to do to him what you would not wish him to do to you. Besides, everyone hates and fears a backbiter; because as he brings to you a bad story about another, he will in the same manner bring to someone else a bad story about you. It is certainly an honor to be able to say of a person: "He never has a bad word of anyone"; while on the other hand, he must be a despicable creature who never speaks of others except to censure or revile them. Never listen to a backbiter, detractor, or slanderer--it is sinful." The Baltimore Catechism, eight commandment. (cf. Sir. 21:28)