Steelwolf correctly pointed out that a GPS chip in all US military would ensure that no prisoners would ever be taken.
Well, you can't make a signal only go to the good guys. I would hate to have the bad guys be able to locate our troops with a simple DF set. Also, it would have to have batteries - 1 more thing to carry. And if it had an off switch the captors would just turn it off.
They kill all prisoners anyhow. A very quick rescue would be their only hope.
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.