Can I use GPS and GLONASS on an iPhone 6S without a wireless network? I have a hiking trip coming up and I was wondering whether the iPhone 6S can be used for GPS tracking if there are no cellular networks available? Or does the phone not have the same GPS/GLONASS support that you would get from a hiking GPS device?
You would be able to do this - in as much as the co-ordinates can be fixed from the satellite.   You need to be aware that it will take a long time to get the fix if there is no reference point to help it (eg known WIFI or cell towers), as it needs to download the almanac from the satellites.

You also need to know that you won't be able to get maps or anything, and mapping may or may not work depending on your application.

Lastly, be aware that, historically at least, phones and similar devices have not had the sensitivity of dedicated devices, so they may not work as reliably.