It happened again today...

 

For anyone who has this issue in the future, it's a quick fix and you don't necessarily have to pull your liner. There's a rectangular cutout on the right side that you can stick your hand into. Reach down and push a small pin towards the front of the car (leftward) and this should "unlock" the fuel door. I'm guessing that it gets stuck sometimes if it's dirty or not lubricated.

 

I had the same issue.. the actuator is going bad. I had to pull the lining back from inside the Trunk (remove one plastic fastener). From there you can reach in and at least pull the locking tab back so you can open the gas door. You can easily remove the actuator (one plastic nut inside the gas door - hand turn) and remove the actuator from inside the trunk. I tried cleaning the movement w/ wd40 and such but it still was wonky. Found a replacement part for $130... replaced in less than 3 minutes total. Now works just fine. Remember the Fuel Door actuator locks and unlocks with every door lock ... so it can wear out.. (I have >105k miles on my G37)

Good luck

 

Spray some lube in there