You'll need a cell signal to use google nav. All gps does is communicates with geosatellites and retrieve your position by coordinates (longitude and latitude). In order for us use those coordinates, we need mapping programs to map the coordinates to the street we're on. Without maps, all we have are just coordinate numbers.
In the case of google maps/nav, we use the cell signal to download maps as we go. Without it, we'll have no maps to map our position.
In the case of garmin or other standalone gps units, the maps are stored in the device for you to use at anytime. That's why they don't need a cell signal to operate.
Like Airmaxx said, there are apps in the market that have offline maps - meaning you're downloading the maps and storing them in your SD card/internal storage for offline use.
