It is permissible to do Gheebat in situations where the primary purpose is to save a person from coming to some sort of harm. For example, a person comes to you and asks that someone wants to marry his daughter, and asks you if you know what kind of person that man is. If you know something about that person which is important in relation to this marriage proposal, then if you tell that person about it with the intention that he and his daughter will be saved from coming to harm, then this is not included in Gheebat.

Just like it is not permissible to do Gheebat of a pious person, it is not permissible to do Gheebat of a sinner. 

Hazrat Thanvi (may Allah Ta’ala bless him) said;

“If a person commits Gheebat or some useless conversation in front of a Salik (a disciple on the path of purification of inner-self) and he does not have the power to stop him from doing so, then he should leave that sitting, and he should not care about hurting the other person’s feelings, because not harming one’s own Deen is far more important than hurting someone’s feelings. If he is not able to leave that sitting openly then he should make some excuse to get up, or change the topic away from Gheebat.”