Nehaji,
If the property is already transferred in your father's name there would be no problem. But still as a matter of abundant caution I would suggest that you pay the proper stamp duty as per 1989 rates and ask for pardon of the penalty and interest. The governent has many times in the past come up with amnesty schemes wherein such documents were registered without penalty and interest.
If the property is not transferred to your father's name then it will not be possible to transfer in your father's name unless the gift deed is registered.
S Jadhav