The Catechism of the Catholic Church notes (para. 2479), both calumny and the related sin of detraction (revealing another’s sins to a third party who does not need to know about them)
destroy the reputation and honor of one’s neighbor. Honor is the social witness given to human dignity, and everyone enjoys a natural right to the honor of his name and reputation and to respect. Thus, detraction and calumny offend against the virtues of justice and charity.
Gossip is a form of either calumny or detraction. People think of gossip as a venial sin but it can be a mortal sin.
The Catechism says (para. 2484)
The gravity of a lie is measured against the nature of the truth it deforms, the circumstances, the intentions of the one who lies, and the harm suffered by its victims. If a lie in itself only constitutes a venial sin, it becomes mortal when it does grave injury to the virtues of justice and charity.