computing the GDP of the larger developed nations is a very large calculation which requires a lot of guessing... take for instance variables like the size of the underground economy (which is included in GDP). funny how these numbers are then treated as an absolute truth. for a large set of countries (like, all the un countries) you cannot have comparable data. even with standards like the system of national accounts which theoretically defines how you compute macro aggregates, there are always variations - the US being one of the countries interpreting the SNA rules the most liberally. the only thing you can hope to get are time series for one single countries consistent over time.