Fiah, the problem I see is, that yes, that's the GDP of those areas on Earth currently, but those GDPs are the result of the current and past economic policies of the governments and the history of those areas on Earth. The whole point of having a nationstate is to make your own country with your own policies, which means that you can't base your GDP off the real-life territories.
Plus, there doesn't seem to be any sense of time in the game. Your nation just grows in population and changes over time depending on your policies, and exists in a modern context, but only some of the things in the game follow time, others (like technology) don't.
I don't think any way of calculating the GDPs of a fictive nation can claim to be realistic. Even if you had all those policy options, they would give different results in different places, depending on the history of your country, what natural resources it has available, its culture and mentalities...