Loan to be returned at the rate of gold

I was 19 years old and my uncle gave me money which he said I would need to return at the rate of gold. So basically if for example he gave me 100 PKR - he would consider it as an equivalent amount of gold on the day he would loan me the money. The loan for studies and was given over a period of time. In the monetary terms, I have paid him twice the amount and the loan is still outstanding. Someone said that in Islam it would be considered as an interest (riba) in Islam!