When you use Google Maps on an iPhone, it will download map data and whether you pay for that data per MB or monthly depends on your data plan.

You do not pay for data received from satellites, i.e. GPS data. I don't know about the iPhone, but my Android can even still use GPS in airplane mode, when the 3G/LTE radio and the WiFi are disabled. The GPS receiver is a passive part that only receives and is unrelated to the regular wireless network, except that having access to the wireless network speeds up the initial satellite lock because the phone can download satellite position data off the internet.