In Arma 2, the GPS was visible on the map and displayed the GPS coordinates of your soldier. Unfortunately, in Arma 3, this functionality is gone, and the GPS is no longer visible. To see your exact position, you have to hide the map, switch on the GPS, and then switch back to map.
It would be great to see the GPS on the map again, showing your exact coordinates.
Note: All of this of course only applies if you actually HAVE a GPS in your inventory.