Gift Deed signed by Fraud

My grandmother is 80 years of age and a person duped her into signing a gift deed to his name and got it registered. We came to know much later about it and we went through the details we found that a lot of the clauses in the gift deed are incorrect and have false claims. Even the name mentioned as the witness to the gift deed has not signed it. The background of this gift deed is we are the landlords of a flat which was given to our Tenants 40 years back on Paagdi System and later when we signed a redevelopment deal with a builder we gave away full ownership rights to our tenants in monetary exchange for our remaining property rights as per the Pagdi System. They got the flat transferred and registered on their name too in July 2017. Later we found out that in April 2017 a fraudster who claimed to be my grandmother's well-wisher and had promised to help with the tenants had made her sign a gift deed in his name. Now we are worried about what will be the consequences, the said property gift deed was signed before the property was solely transferred to our tenants. Please help to get out of this situation legally.