GPS works with a fully offline and in airplane mode device. GPS is 100% passive, unless you want faster initial fix, for that A-GPS comes into help and require data connection.
But plain nice old reliable GPS is 100% passive and works fine with totally disconnected devices. Actually original GPS devices (who remmebers those…) where 100% offline. Maps on the SD card, no live traffic or accidents data, nothing fancy. Yoiu had to purchase yearly maps updates too, what old times.
Gentoo! Of course!