"GDP is the standard measure of the value of final goods and services produced by a country during a period minus the value of imports." OECD, Paris
"Gross national income (GNI) is GDP less net taxes on production and imports, less compensation of employees and property income payable to the rest of the world plus the corresponding items receivable from the rest of the world (in other words, GDP less primary incomes payable to non-resident units plus primary incomes receivable from non-resident units).
An alternative approach to measuring GNI at market prices is as the aggregate value of the balances of gross primary incomes for all sectors; (note that gross national income is identical to gross national product (GNP) as previously used in national accounts generally)." OECD, Paris
Because GDP aggregates all economic activities of a country (and by various statistical concepts) it can be found in the core data collections of most sources. There is no national account statistics without GDP, e.g. GDP in total, per inhabitant or its growth rate.
GDP can be expressed not only in national currencies but also in US-$, international US-$, Euro, Purchasing Power Parity (PPP), Purchasing Power Standard (PPS), in current, constant or market prices and so on.
The following chart compares the GDP per capita - expressed in Eurostat's Purchasing Power Standard - of the European Union (=100) to Japan, Norway, Switzerland, Turkey and the United States:
DSI’s National Accounts Statistics App presents the core collection of national accounts data times series for more than 180 countries of the world. It has been compiled on the basis of some of the most important national accounts series as provided by international sources, e.g. World Bank, UN, IMF, OECD.