Na verdade, fiz uma pergunta semelhante (e respondi pessoalmente ) em link porque eu estava antecipando isso.
O showrunner de Gotham Bruno Heller explicou isso ao Entertainment Weekly há algum tempo atrás :
Gotham is not set in any particular time period: Okay, this has been out there a bit, but Heller had a nice quote elaborating on the show’s timeless quality: “It’s a mash-up, to use the modern phrase,” he said. “If today Batman exists, then this world is the past. But it’s everybody’s past, an 18-year-old’s past and a 54-year-old’s past. So in your memory, the past is all mashed up together. So in this Gotham, it’s a kind of timeless world. It’s yesterday, it’s today, and it’s tomorrow all at the same time, because that’s the world that dreams live in.”