"which countries would lose the greatest amount of GDP?" means that you're searching for the capital cities with the highest GDP. You should say something about the percentage lost instead.
The article that contains the image you provided as a 'source' says that it is only looking at data from within Europe. This quiz needs to be remade or renamed.
This doesn't make a lot of sense. While I'm sure removing Paris from France's economy would do a lot of damage, I feel like removing something like Bangkok from Thailand or Kuwait City from Kuwait would be much more devastating
Good quiz by it is only Europe and only larger European countries. Pretty sure if you took Luxembourg City (which contains over 20% of the countries population and clearly a higher % of the nation's GDP ) it would be a higher percentage loss.
https://www.statista.com/chart/11515/the-capitals-economic-power/