Even though it doesn't use 3G service, the 3G devices do have GPS; the WiFi only ones do not. Since most of the GPS APPs use pre-installed maps and location based on GPS, you do not need to use 3G service for it to map your current location.
I use GPS APPs on my phone all of the time when I am completely out of range of any service and it still works fine. I even took out my SimCard once to see if the APPs will still work without it, and it did.
The Skiing APP I use, SkiTracks, does an excellent job of mapping all of my runs when I am skiing at resorts where there is no service of any sort.
When I use the RunKepper APP I turn off my 3G antenna to save the battery and so I do not get calls interrupting the APP or my music while I am working out. Still maps my workout perfectly. The weird thing is if I turn of the WiFi antenna, then I will lose the GPS signal.