This is where I remain confused. The Google Nav website advertises:
"Offline reliability - Get continuous guidance and automatic rerouting, even if you lose your network connection after you’ve begun a route. "
What does this mean if you don't have a route loaded in the phone?
It means very little. There is a setting "Cache map tiles in advance" or something like that, but it only caches the tiles (which are a lot smaller than Garmin map chunks) that it thinks you're going to need, based upon where it thinks you're going to go. Since you can't use a pre-generated route, it simply does the direct route, and preloads those necessary tiles.
Going in, I hadn't realized there was no way to use googlemaps routes. I could see them, but not get the Navigation app to recognize them or follow them in any way- so for that reason the basic google navigation was less than completely helpful.
It worked fine for a "get me to this address the quickest way" use, but that was it.
The app I wound up using more was "NavFree". It couldn't accept generated routes, and I don't believe the app itself has the functionality to generate them, but you can preload all the maps you want in a state by state fashion. In my case, I loaded California, Nevada and Arizona. For the typical "Get me to this address" or POI, it was phenomenally good. If I could figure out how to do routing, I would be 100% happy with this app.