GPS as aGPS needs a very small amount of data for to has a little faster GPS fix. But if you switch of the (3G) data, GPS will work without data.
Google Maps let you pre-download maps of six areas, with about 80MB each, as offline maps.
But note, for to compute or re-compute a route for navigation, Maps will need a data connection.
But there are offline turn-by-turn navigation with pre-loaded maps. These apps, like Navigon or CoPilot, don't need any data connection at all for navigation or compute a route.
Harry