I'll be going for a hike in the mountains this weekend. I won't get cellular or data, but I will get GPS. Will pokemon still appear nearby or do I need to have data access to see them?
They will not unfortunately, you need an active internet connection for both the spawning and the catching of pokemons.
From my experience as soon as I lost data signal the game would popup saying it couldn't connect anymore and I was unable to continue playing. So unfortunately it looks like a constant data and GPS signal is required.