"all iPads have GPS..."
Unfortunately, not true. Only the iPads with 3G/4G cellular capability have the chip that receives the signals from GPS satellites. The GPS function does not require an active connection to a cell tower, or even a contract with a cell phone company.
The WiFi-only iPads have a way to calculate your approximate location by the strength of WiFi signals from routers near your position. If no Wi-Fi routers are within range, then you have no location information.
An external GPS receiver will work with WiFi-only iPads. It's less convenient to deal with the external device, but you can place the receiver in a location where it can see more GPS satellites. The result is better accuracy. For example, you can place the receiver at the base of your windshield, while the iPad stays in a mount on the dash.
That said, most people who have an iPad with internal GPS receiver report that it works OK inside their vehicles.
As a practical matter, the external Bluetooth GPS receivers cost about $100, and the 3G/4G iPads cost about $130 more than the WiFi iPads do. I think the extra convenience of the internal GPS is worth $30.