Well, seems the design of the new Z is pretty solid if the complaints are becoming so petty.
First, people complaining about wheel offsets because Nissan tries to give us big enough wheel wells for the inevitable super wide wheel/tire setup. Now it's about a gas door? TF??
As to the reasons behind it, there is none. There have been a few automotive journalists posit about it, turns out to be arbitrary. On some cars it is on the left, others on the right. Some cars there will be an exhaust pipe on the opposite side of the filler pipe. Most unibody cars have the fuel tank mostly centered. A fuel tank can be heavy when full, and on a sporty car it's better to keep it in the centerline of the vehicle. The only way I'd complain about a certain fuel filler door is if it interrupts nice body lines and/or is larger than usual, as is the case with some DEF Plug-In Hybrid vehicles.