What are the ways to cancel fraudulent gift deed?

My Grandpa was deceived by my cousin that we need to register WILL with registrar. So my Grandpa, then 95 years old, signed documents of gift deed assuming he is signing WILL, in which he granted equal right in property to all 3 sons. But in reality my cousin managed to get him to registar office and got all the property transferred in his name by putting Gift Deed in front of Grandpa. This happened in 2014. He didn't change names on Electricity, Water bills. Even he didn't change name on Corporation records (Ghar Patti) for all these years. Last year my Grandpa died. When my grandpa's advocate was called he showed actual WILL in which equal share in property was granted to all 3 sons. When Advocate moved documents to get house property transferred in the name of all three sons, he got to know that on City survey records, property has been transferred to my cousin. Now we got to know that one of my uncle was party to this fraud. What are the ways to us to get that property back?