What i have learnt from scholars on how to cure this disease(yes it is a disease) is to
-impose some penalty (financial and in the form of offering nafl salat)on yourself for doing gheebat, make the amount of penalty higher untill you stop making gheebat.
-Communicate to the person that i unfortunately made your gheebat and i am sorry for that, that will put a dent on our nafs and inshallah will keep the nafs under control.
-Straight on refusal to some one who want to do gheebat is painfull at start but after that , you will feel much better and relieved.
Just keep the gunah of gheebat in mind and then inshallah it will easier to refrain from it.