I use GaiaGPS as well on my iPhone 6+ for all my hiking tracking/route/off line maps.
Works well for me and even has an Ample Watch app so I can check my progress without using the phone.
You do have to make sure you have space for all the maps you want to download. I have 64GB, so never really had a problem downloading what I needed. I actually download USGS, Openhiking, and satellite maps. I find the later sometimes helps when I get off the trail (intentionally or unintentionally) to find my way home.
You do need to keep it charged, so a extra battery pack of some kind is a good idea for longer hikes. GPS does eat power when used continually. A day hike usually isn't a problem (I keep a small battery in my day pack just to be safe), but for overnight tips I have a bigger battery.
I'm actually thinking of getting a solar charger to help for my two planned 5 night hikes this year, plus my 6 night Japan Alps trek. Probably the Instapark Mercury 10M Solar Panel Portable Solar Charger.
