"Turn by turn navigation is indeed a very nice feature, and we'll continue investigation to whether other services (other than Google Maps) allow creating commercial applications that simulate this feature."
Have you found anything since then? There are quite a few Android application (Here, TomTom, Waze etc.).
Can any app support GPX files which I export from Fnc Maps?
After further investigation, creating turn-by-turn navigation is prohibited by many of the supported services license agreement to create commercial applications.
GPX files can be exported and they follow a standard, thus importing them in other third-party tools should be possible.