1. The gift deed once executed , registered ad acted upon can not be revoked unilaterally by the donor.
2. So in your case your grandfather after his fit deed to his son and daughter can no more deal with the property and hence the deed of revocation executed in 2012 is a non est and not binding on you.
3. So on death of your father you inherits his half share in the property along with your other co -sharers, if any and if you are refused to be handed over the due share you can file a suit for declaration that the deed of revocation is null and void and not binding on you and for partition to get your half share in the property.
4. So meet a local advocate and file the suit in local court.
All the best.