Thanks for your replies, guys ! Yes, I thought it's odd as well. I was surprised that there's no spare . Found out a bit information from Infiniti G37 website but for convertible one:
"If you have a 2009+ Infiniti G37 Convertible and you happen to live somewhere other than Maryland, then you probably have a Tire Repair Kit in your trunk instead of a spare tire. Why is it this way? No idea. Someone within Nissan made the decision based on space, weight, or cost." ?!?
Just PMed GeeDog, hopefully the 350GT spare would fit.