As others have said, Google Maps by default includes low-detail maps for the entire planet, so even without data coverage you should be able to get a reasonable idea where you are. However the detail isn't good and they don't include locations for many landmarks. eg, I just checked New Zealand, and for the entire country the low-detail maps include 5 cities! You can test it yourself - just put your phone in airplane mode, and then scroll/zoom to somewhere that you haven't been recently and check the detail.

The other thing to keep in mind is that GPS doesn't necessarily work well on aircraft. If you're not by the window then there's a good chance you won't get a lock at all. If you're on a 787 then even if you are by the window you almost certainly won't get a fix. I use an app called "GPS Test" which shows you the satellites that the phone is connecting to, and then only once it's got a fix do I jump over to Google Maps.