GPS: I keep stumbling over this in books, films and on The TV and not only is it jarring, it's gonna get some tenderfoot killed dead if they don't knock it off.
A GPS device knows where it is (if it can receive signals from enough of the satellites). It cannot tell others where you are. Can not.
There is no transmitter in your handheld or dash-mounted GPS. It's not talking to the Global Positioning System Satellites; all it does is listen to them and then do some math. (As far as I know, the only people talking to GPS birds would be DoD and last I knew, that was in the form of admin-type stuff not in-the-field position info).
That said, there are ways to hang additional hardware on the GPS device and use that to send the GPS position to others; hams have been doing it for years. But it takes a transmitter and most of 'em are power-hungry. It is limited to the range the combination of frequency, power, antenna system and propagation conditions permit.
Your cellular telephone can be tracked back to the particular cell it is in and if your phone includes a GPS receiver, it can send that info, too. This is smaller and more power-efficient than the APRS alluded to in the previous paragraph but if you're in an area where there is no cellular coverage, oopsie! She no work.
(Update: Joe Huffman on phone tracking, privacy and how caution and even paranoia can't stop stupidity).
And again, the first thing is the GPS receiver has to have a lock on enough of the GPS satellites to be able to figure out where it is; in tunnels or between tall buildings, it may not work so well. Or at all.
So, writers, will you please, pleeeeeeeze stop breezily tossing "GPS tracking" into your work in the form of Character holding up GPS Receiver and remarking, "They can track us with this," 'cos they can't. Now, that iPhone you've been taking for granted, on the other hand....
(Side note: the slick way to write this for most adventure-type apps is to use a transponder, something that is polled and spits out the current or last-known-good GPS position, suitably flagged. This is economical of batteries, plausible, and darned hard for anyone who doesn't know how to poll it to locate using normal bug-hunting tools. For extra credit, use some form of decent data encryption and/or spread-spectrum; it can easily be about celphone-sized).
(P.S. GPS receivers can be spoofed. It's a neat trick. Uncle Sam knows how and he's not talkin').