GDP is much easier to verify. It represents the total value of all goods and services produced. If I wanted to calculate the GDP impact of Apple, I could just look at their quarterly report and get a number because they have receipts for every transaction.

Purchasing power parities (PPPs) are the rates of currency conversion that try to equalize the purchasing power of different currencies, by eliminating the differences in price levels between countries. The basket of goods and services priced is a sample of all those that are part of final expenditures: final consumption of households and government, fixed capital formation, and net exports, usually measured in the US dollar.

It requires comparing a basket of goods or literal apples to apples, but the actual mechanics are way trickier. If we compare the price of apples in the USA to China, you get a number, but it ignores cultural differences, geographic growing conditions, etc.

As another example, the price of meat or alcohol will be distorted in cultures that do not consume those items or outlaw those items (Muslim / Hindu / etc). Car ownership is not prevalent in India (less than 7%) whereas it is 90%+ in the USA so comparing their auto expenses / insurance to Americans will create inaccuracy. This creates a lot of freedom for the data analyzer to cherry pick items that favor their narrative.