This is a "hard money to stocks" ratio. The normal S&P 500 is essentially a measure of how much cash do you expect American companies to generate relative to competing investments (represented as the discount rate). By pricing it in gold, you tease out the effect of dollar devaluation and inflation. Some fraction of those earnings will be because the dollar is worth less and so companies can charge more, but this affects both future earnings and the dollars you use to buy the stocks. If you measure in gold used to buy the stocks, you eliminate the denominator there, teasing out present inflation.

That the ratio is not as high as the raw S&P 500 indicates that a significant component of the S&P 500's high valuation is because investors consider the dollar to be worth less, and expect that it will be worth even less in the future. However, that the ratio is better than it was in say the 1970s suggests that investors believe there is still a significant amount of real productivity in American companies and it's not all inflation.

One key assumption here is that gold = hard money. If gold is considered a speculative asset like Bitcoin (again assuming Bitcoin is not hard money, which is uncertain in 2025), one which is traded only by a small number of enthusiasts, then the ratio may measure speculative fluctuation in the value of gold more than any useful quantity.