Reverse geocoding (country) from geofield with coordinates
this is a question that I asked to myself some time ago. I suggest to start from the geocoder module.
This module "translate" an address to a lat./lon. parameter that could be displayed in a map.
Reversing this process - if I understand correctly your question - seems to be a WIP: actually, no out-of-the-box solution.
But you can check and try the suggestions in this discussion.