The process of getting an arbitrary address from geo coordinates is called "reverse geocoding." Google it, and most of the links you'll find basically tell you to ask a web service to do the look-up for you. Know that geocoding can be inexact, so your results may not be 100% correct.
I'm working on a solution right now where I have to find the address for a set of coordinates, but only from a list of known addresses. I found the coordinates for all the addresses in advance using non-reverse (forward?) geocoding (i.e., asking Google Maps where these places are). When the user needs to associate a photo with an address, they tap a location icon, which launches a script that uses the Location() function as the basis for a "geobox" find for addresses near that location; the user then selects the correct address. I don't know if that approach might be useful for your particular application, but I thought it might be for someone who finds this thread in the future.
You should have a look at the Contacts Starter Solution of FileMaker Pro 12.
You'll find 2 scripts that will do the trick :
Find my location | Get GPS Coordinates
Get Reverse Geocoding Work Address | iOS
You'll also need a few Custom Functions available in the template as well.
If you do not want to use Google, you can also have a look at http://nominatim.openstreetmap.org
gotta love that!
that's breakfast for databases!