Back when I had an iPhone the maps app (Google-based at the time) cached a certain, limited amount of info. This was great as you could then turn off mobile data service, and enjoy GPS-guided maps for free while abroad.
Now I have a (vastly superior in every respect
) Windows phone, which has an integrated app where one can choose to download maps for offline use, per country. there's probably an app for iOS that will do something similar.
As long as you are not flying above FL600 and faster 1000 knots groundspeed, the GPS should function perfectly
