I do not think that gossip is necessarily a mortal sin; it, like any other mortal sin, has to meet the three tests of a mortal sin.
In other words, if the matter is not serious, and there is not a serious intent to harm, then I fail to see how gossip could be defined automatically as a mortal si.
It is possible to define it in such a way that it is always a mortal sin, but then would have to re-define what else is often called gossip, as the rest of the loose talk would have been defined out.
Having said that, it is a sin that is all too easy to engage in and not make an accounting of later.
An old, old story has the town gossip going to confession and confessing the sin; the priest, who has counseled the penitent many times previously feels he has been ineffective, so he crafts a new penance in hopes of getting the penitent’s attention. He tells them to teak a feather pillow and go up in the tower on a windy day and release the feathers, and then to go collect all of them.
The penitent protests that is not possible. The priest then tells tehm that it is easier to retreive the feathers than it is to undo the harm, and recover the gossip and repair the damage to the reputation of the one gossiped about.
Food for thought.
