I think some people have watched way too many hollywood movies. A GPS 'chip' would have no value at all. A chip of this sort would not work inside of any structure. In order for it to be workable, the device would have to be quite large. Second, it would have to be combined with a transmitter in order to broadcast. A device of this type would be the size of a small cell phone. Do you really want that implanted under your skin?
There are other ways to do this type of thing but a GPS chip is not one of them.
What are they, and why aren't we doing them?