GPS signals are incredibly weak and are easily disrupted. According to Garmin's GPS for Beginners (500 KB PDF file):

Each satellite transmits low power radio signals on several frequencies (designated L1, L2, etc.). Civilian GPS receivers "listen" on the L1 frequency of 1575.42 MHz in the UHF band. The signal travels "line of sight", meaning it will pass through clouds, glass, and plastic, but will not go through most solid objects like buildings and mountains.

To give you some idea of where the L1 signal is on the radio dial, your favorite FM radio station broadcasts on a frequency somewhere between 88 and 108 MHz (and sounds much better!). The satellite signals are also very low power signals, on the order of 20-50 watts. Your local FM radio station is around 100,000 watts. [Actually, most of them are much lower power than that.]. Imagine trying to listen to a 50-watt radio station transmitting from 12,000 miles away! That's why it's important to have a clear view of the sky when using your GPS."

I have 3 GPS receivers, two portable (one Garmin II+ and a Lowrance AirMap 100) and one panel mounted model (Garmin GNC-300XL). If I turn on my portable units indoors and away from windows, they seldom can lock onto the satellite signals. That's in an ordinary wood-frame house. Both of the units have pretty good antennas (probably a lot better than any implantable antenna could be) and they still have trouble indoors. I have not tried them in heavily wooded areas but there could possibly be disruption there, too. That limits the places where a GPS implant could locate itself.