Moreover, you can't get "real GDP change" from just enumerating the goods and services traded and their prices at the initial and final times, if these are far away enough to have encompass at least one business cycle.

So you have to fall back on "real GDP change" as a feature of a history of economic development.

By there isn't a sensible mathematical definition of "real GDP" or the "price level" I mean you can't meaningfully compare the "real GDP" or the "price level" between two points.

Most economists teach a theoretical framework that has been shown to be fundamentally useless. -- James K. Galbraith