In a country with a non-gold based paper money system, GDP/GNP is more a measure of the amount of spending, and is related to the total amount of money in the economic system, rather than the total physical volume of output of the economic system
“In a country with a non-gold based paper money system, GDP/GNP is more a measure of the amount of spending, and is related to the total amount of money in the economic system, rather than the total physical volume of output of the economic system”
In the gold poor world of colonial Virginia physical output exactly equaled the money in their economy, the money being tobacco receipts for 100-150 years. The Real Bills doctrine in banking is a similar, non-agricultural idea.