The facts stated are not convincing. As per my understanding Uncle has given to your GM a gift, and she is no more now. Your uncle as she is no more has filed a declaration suit for cancellation of gift deed. Your father wanted that as it is your GM property already, self-acquired through gift deed it should be distributed to legal heirs of your aunt. When your uncle has already filed a declaration suit, you can only prevail for the continuation of the gift . Where is the issue of unsound mind? Who are her legal heirs and how the property was enjoyed by her since the date of the gift. Those who looked after your GM has extracted any will from her ?
My Grandmother who is 80 Years old paralyzed, so in the due course what my uncle did is took my Grandmother without knowing other legal heirs and made a part of Shops of the property as a Gift Deed on to his name.
Your uncle has filed a declaration suit, argue before the court and establish will was a fraud or obtained through undue influence. In fact, your uncle has made your job easy, argue and get that gift deed canceled by providing all those medical reports or witnesses statements etc.,
Is the gift deed registered? Now that the uncle has filed the suit for declaration onus rests within him in proving deed. Your father can only deny the averment of gift. As the property belonged to the grandmother, it was not ancestral. She was competent to make the gift.