Hadhrat Thanvi RE further said;

“Along with apologizing, it is also necessary that you praise the person whose Gheebat you had committed, in front of the same people in front of whom you had committed his Gheebat, and clarify that you were wrong.”

This is because you have apologized to him, and have also done Istighfar in front of Allah Ta’ala, but the people in front of whom you had committed his Gheebat are still carrying that bad impression from what you had said. To negate that bad impression, now you should praise him in front of the same people and admit that what you had said may not have been completely true. 

Hadhrat Thanvi then said;

“And if what you had said wasn’t wrong, it was true, then say something like, ‘please do not start doubting that person because of what I had said because now even I am not 100 percent sure whether what I had said was true or not.’ This will be توریہ because one cannot be 100 percent sure about anything without Wahy (Divine revelation).”

It means that if what you had said was true, now you cannot knowingly say that it wasn’t true. Rather, say something like “do not start thinking bad of that person because of what I had said because now I am also not sure whether what I had said is 100 percent true or not.” For 100 percent certainty one needs either direct observation or divine revelation. There is no other way of having 100 percent certainty. Therefore, it is okay to make a statement like this. Insha’Allah it will atone to some extent for the Gheebat you had committed about that person.