Is the father obligated to return the gold he borrowed from his deceased wife to their children, and how is the inheritance divided among the father, the three children, and the grandmother (the mother's mother)?

If your father truly gifted the gold to your mother as a genuine transfer of ownership, then after her death, it becomes a right belonging to her heirs. The debt your father borrowed from her must be returned to the heirs and divided among them. The husband receives one-fourth, the mother receives one-sixth, and the remainder is divided equally among the three children. The estate is divided into 36 shares: the husband receives 9 shares, the mother receives 6 shares, and the three children receive 21 shares (7 shares for each). The matter must be referred to the Sharia courts to verify all bequests, debts, and rights before distributing the inheritance.