GPS will work - GPS devices (like Magellan) don't have data or wifi connections. GPS is its own radio system, its own radio and its own antenna.
GPS apps probably won't work without a data connection, unless you download the map when you have a data connection and stay within the area of your downloaded map. The phone has to get the map from somewhere. (Driving directions are given by latitude/longitude. Maps looks up the address in a database [using a data connection], converts it to a lat/lon, then gives you directions to that, so if there's no data connection at all, you can follow the downloaded map, but you probably won't get.turn-by-turn directions, because Maps doesn't know where you're going - you'll get an error of some kind. If you set the destination up in a hotel with wifi, then don't have a data connection, it'll probably work.)