iPhone app to follow/navigate an imported .gpx
Gaia GPS is what you're looking for. Costs $20 USD on the App Store, but the best at what it does.
I don't drive, ride, hike or use GPX myself, but here's an idea, using TomTom:
Get GSAK (thanks to tomtom's forums) to convert the GPX to a TomTom file format and add it (thanks to groundspeak forums) to your iPhone's TomTom.