Hello, I'm Carl.
For APRS tracking, you need: (1) a 2m FM radio that can transmit on 144.39 MHz. I assume a Retevis (never heard of it) can do that.; (2) a GPS receiver that can output your latitude & longitude in NMEA 0183 format (Is that what you meant to use the iPhone for? I didn't know it could do that); (3) a TNC such as a Byonics TinyTracker or Kantronics KPC-3 that can accept the NMEA location data, change it to an audio signal, feed it to the radio, and control the radio; (4) a separate antenna for the APRS radio; and (5) cabling to connect it all together.
Some Kenwood radios, and probably other brands too, have all of this built into the radio -- even the antenna. I've never owned one of those, but I have built and run several trackers using simpler (read: cheaper) Kenwoods, Byonics and Kantronics TNCs, and Garmin and Byonics GPSs. There are several lying around the house (RV) someplace.
What a tracker does, whether it's all one fancy piece or several simpler pieces, is transmit an FM packet from time to time containing your callsign, location, and sometimes other info. There are APRS receiving stations almost everywhere that will receive your packet and (via Internet) send it to web sites like www.APRS.fi
(it's in Helsinki). When your friends wonder where you are, they go to the web site and enter your callsign to show you on a Google Maps display. You can use www.APRS.fi
in other ways, too, like displaying all the APRS stations in a particular part of the world, or displaying your track for the last, say, 24 hours. It's really slick