[Minato Ku]

Be careful this kind of subject can quickly deteriorate.

For European cities, this is rather a list of the GDP of the region around the city than the metropolitan area.
Rhein Rurh Figures are for the whole North Rhine-Westphalia lander, London figure is for the whole South East corner of England
Naples figures is for the whole Campania region, Rome figure for the whole Lazio region, Milan figure are for the whole Lombardy region.
Some cities have region that are approximately near the size of the metropolitan like Madrid or Paris but most have not.
It is better to use data on a lower administrative division like the Provinces for Italy or counties for UK. It would lead to much accurate estimation.