I use Gaia also on an i-phone 6. On airplane mode it only updates your location when you hit the update button, and therefore is not constantly dinging your location and battery life. I hunted north of Durango CO in 2015 and could use the phone as a gps in that way and end the day with 80% battery life. I used an external battery charger and had no problem going 4 days at a time away from electricity. Ran out of water before we ran out of charge.
Gaia cost $20, you can download maps ahead of time, and add layers, trails, and waypoints (where you are currently, or locations you haven't been yet) to the map.
I had zero problems using the phone as a stand alone gps.
