Does a deceased grandfather inherit from his son land that the son gifted to his own son and registered in his name, and then the son died before his father?

It is not permissible for a father to favor one of his children in a gift unless the others agree. If a father gifts his son a valid gift and the son takes possession of it, it becomes his property and that of his heirs after him, and the gifting father's right to reclaim it is forfeited upon his death. If the original gift from your grandfather to your father was valid and just, and then your father gifted it to you with a valid gift, and you took possession of it during his lifetime, then it is entirely yours, and your grandfather has no right to inherit it. However, if the gift was not valid, or you did not take possession of it, then it remains your father's property, and his heirs, including your grandfather, will inherit it. Justice is obligatory in giving gifts, and whoever has been favored should return the favor.