The gold interest rate earned on fiat gold is commonly referred as the gold “lease” rate.
It is calculated as:
Gold Lease Rate = Libor Rate - Gold Forward Rate.
The LBMA presents the data every day at this link.
Whenever the gold lease rate tops out (spikes upwards), the gold price will hit a bottom as central banks demand the gold back from the bullion banks at higher gold lease rates. So it is a bullish sign to have high gold lease rates. It means that the GOFO rate is very low, which indicates backwardation in gold.