Is the father required to pay the value of the gold in money or in grams of gold after the increase in the price of gold over time?

The father is obligated to return the equivalent of the borrowed gold, even if its price has increased. However, if he borrowed the price at which it was sold, then he is only obligated to return that price, because a loan requires the return of its equivalent. There is no consideration for a change in value when repaying debts.