Is it permissible to pay off an old debt of gold in grams with the same number of grams of new gold, without a prior agreement on the type of gold? And is this considered a good repayment or a loan that brought a benefit?

There is no objection to exchanging a gold debt for another gold of higher value if they are equal in weight, provided that this is not a stipulated condition, based on the Hadith: "Indeed, the best among people are those who are best in fulfilling obligations."