When you buy a gps unit...?

No, you don't need a subscription. In a way, you've already paid for it with your taxes. The govt operates the GPS satellites.

And despite what some people believe, the government cannot track your GPS use- it's a one way signal. Even though it seems like the government must know your location in order to tell you how it works, it's actually the receiver that determines its location by measuring its distance from the satellites (by measuring the difference in timing between the signals). So your privacy is also safe.

There are systems that let you track children, etc, by GPS, but they work by adding a cell phone into the unit so that it can upload the location data to a cellular network. But a standard handheld device is one way only.