Real means inflation adjusted.
I calculated the real interest rate by subtracting year-over-year CPI from the current 3-month T-bill yield.
One could also use the Fed Funds Rate or 1-month T-Bill rate as the yields are all about the same.
The following chart puts the negative interest rate theory to the test,
Real Interest Rate vs Price of Gold
The chart above shows the monthly gold close, not monthly highs. That explains why the 1980 top does not show.