Google Earth must be connected to get data. It will not work unless you are connected. Openstreetmap.org has links to some apps that use it and at least claim to work with no connection. I have not tried them yet. If you do I would be interested to hear about your experience.
You would need to download and cache all of the maps necessary for your route before you get out of WiFi range. If your GPS software supports that, there should be an explanation in the settings for it.
Locus Free works pretty well with a bluetooth GPS unit or any built-in GPS. You download the maps you need and then you can navigate and zoom as you need with GPS available - all offline.