California.
Any contract using value besides the US dollar, as debased as it is, would be a hard sell. A seller's principal would be protected if the contract required payment in gold, dividing the principal financed by the price of gold at contract date, That does not protect interest and principal monthly payments should there be an reset, devaluation or a "new dollar" introduced to solve the massive debt problem, or a stampede of investors out of markets crashing the financial system.
On the other hand, the deed of trust enable foreclosure in the event of deflation and economic depression where the buyer is unable to make payments in an economic downturn with no inflation.
So, there is a risk to both parties depending on what happens in the economy.
If one can tie a contract to some price index, seems to me one could tie it to gold.