Is it possible to tack anyone's cell phone through GPS?
No. It's not possible to "tack" anything "thorough GPS".
The GPS system is transmit only - the GPS satellites don't receive anything from your phone, your car, or any other GPS receiver.
Interestingly - that's why they are called "GPS Receivers" and not "GPS Transceivers" or "GPS Transmitters"
Because they receive only.
To track the position of something, that "thing" needs to receive GPS signals, calculate its position, and then relay that position somewhere else - for example a web site over a cellular network.
You can potentially track someone's GPS through their phone, but not the other way around. As was said, GPS devices are RX-only.