Where is Ergastulum located? In the anime, Ergastulum has the architecture of a typical Italian city. Yet, the newspaper is printed in English and the cars drive on the right, although road signs and the like do have a European feel to them. The police cars are also reminiscent of LAPD cruisers, in that they are black and white. Finally, from looking at the currency depicted in the anime, it seems that the city prints its own money.



I do remember a scene where a newspaper article shows something happening in Paris, France. So I do think the story takes place on Earth.

I have looked over the Gangsta Wiki and so far it does not seem to have any information on where exactly the city is located. Do we or will we know where Ergastulum is located? Is it its own country?
The setting is generally set somewhere in France or Italy since both people from the Mafia and Perrier (a sparkling water drink from France) are common. Worick also says 'ciao' before killing a gangster, which is an informal Italian word that can be used as a goodbye. 

As you have pointed out, there is the newspaper article that states an incident occurred in France. There is also the fact that people call Nicolas an 'oriental' which is what Asians are commonly called in Europe, so it is likely set somewhere in Europe. Also, remember that France and Italy share a land border, and so are very close to each other.