No this cannot be considered as the ancestral property, yes you challenge the gift deed and all the legal heirs of the grand mother that is father and his siblings shall have equal share if the gift is made when her consent to it was not there or she was not in position to give consent, If anything fraud or cheating is done then only criminal case can be filed in this case you have to file a civil suit for cancellation of gift deed and claiming of your share on ground that grandmother was not in position to give consent. Further your father has share he has to file the suit.