For some countries, it is probably impossible to count by GDP. GDP is only one-year long indicator, when Poland lost 38% of total wealth, over 50% of industry, Warsaw reduced to rubble (85% of city, 90% of industry), over 50% of all infrastructure. Poland lost 45% of it's prewar territory to USSR and got about 33% from Germany (it was almost a desert - Red Army stole anything worthy and population fled), there were huge population loss and swap. Poland 1939 and Poland 1945 are basically two different countries.

Official number of GDP lost between 1939 and 1945 is 55%.