It would have been impossible for Greece's GDP to drop by that amount (in absolute terms) before it joined the EU. Because it quadrupled after joining.
It quadrupled after joining because it borrowed money from Germany and other countries to buy German cars and other goods, not unlike the early stage of Bretton Woods. It had the lowest debt level before joining, like a fat cat to be squeezed by EU bankers, which was also why it was fast-tracked in joining the EU when it clearly didn't meet many clauses of the Copenhagen Criteria.
But when push came to shove in 2008, the debts were exploding not in Germany, but in Greece.
GDP stands for gross domestic product. As its name indicates, GDP measures the production of final goods and services. Borrowing foreign money to buy foreign goods has no effect whatever on a country's level of production and therefore it cannot have any effect on the country's GDP either.
> Borrowing foreign money to buy foreign goods has no effect whatever on a country's level of production
And this is where you're wrong. Borrowing money has a direct and deep impact on ta country's level of production, mainly because that cash is dumped on the economy by spending it on buying goods and services.
But when push came to shove in 2008, the debts were exploding not in Germany, but in Greece.