The fallacy in the equation is making Gold equal to a fixed amount of Currency, instead of the Currency equal to an amount of Gold. Had the latter been set, then the value of the currency would track with the Gold price - but then the banksters couldn't steal with their fiat money games.
Having $20 always equal to one oz. of Gold doesn't mean we have stable prices. Money's needed for wages, for rent, and for food, and it's those prices that matter.
In terms of wages, rent, and food we got a price of gold that jumps all over the place from one month to the next and always being able to buy gold at the same price is irrelevant.
Well, if an ounce of gold is $1000 then $1000 is an ounce of gold. So much for your fallacy.