The GPS (Global Positioning System) is a utility which allows you to pinpoint your position on a Map. When a map and a GPS are in the inventory,  the player's position on the map is indicated by a circle and a blue dot, thus making navigation much easier. They are rare and are only found in military loot spawns.

GPS's cannot be used without a map, even though most modern GPS's have a built-in map. If you do find a GPS and you do not have a map, definitely keep it, but you'll need to find a map to use it.