Most smartphones have built in GPS antennas and circuits like a Garmin, so the GPS side of the function does not use data. What does use data is the downloading of maps, but most heavy data usage is when keeping the GPS function on during long drives where you likely don't need an updated view of Highway 80, etc. If you're worried about data usage on a smartphone GPS, use the GPS function 1) only when you're relatively close or 2) accessing wi-fi (stop at a Starbucks or McDonalds) and then cache the maps into memory.