GPS is NOT dependant on cellular service. I suspect that the WiFi only iPads simply do not have the GPS module in them. As I said, my nVidia Shield K1 is a WiFi only device and it HAS a GPS receiver module. It knows where it is.
My Asus Transformer tablet is also a WiFi only tablet. However, it has no GPS module much like your experience with the iPad Wifi only models. So its device dependent on whether the unit has a GPS receiver or not.
And so does my Samsung Galaxy Tablets...I have 2, both are wifi only and both also have GPS.