Dumb question from a newbie

First off GPSs don't need a cell phone signal to work, they use satellite signals that can be received in any part of the world where they can 'see' the sky, a cell phone signal is sometimes used to download a map of where you are, but you can get around this by downloading all the maps you require to your cell phone's memory before you start your trip

Dedicated GPSs have built-in maps and don't receive cell phone signals at all, so any dedicated GPS or cell phone will do what you want, Open Street Maps often have trails on them and these maps can be loaded onto Garmin GPS units, these units can also display your own satellite imagery and custom made maps