The Phantom contains an internal GPS receiver which it uses for most functions that require GPS data. GPS data is freely available and does not require any type of monthly fee.
If you want to use the Follow Me flight mode or be able to reset the home point to your current location after taking off, you'll need to use a mobile device that contains a GPS receiver. If you require those functions and you're using a mobile device that does not contain a GPS receiver, you could connect one of these external Bluetooth GPS receivers to your mobile device.