This should have been in the market long time ago. Probably it would have, but now we can see the "khullam khulla farokht" of this thing. Well as far as jamming GPS or any other signal is simple. The jammer has to transmit its own signal (noise) at exactly the same frequency that it wants to jam. Its high power signal would interfere with the original signal and will destruct it. Similar technique is used to jam the signals of your mobile.
However, if the device is being used for the reason that he(user) may not be tracked by someone, i.e. agencies, well they can track him in any way. One of the possible options is tracking using mobile as this jammer would only jam the frequency at which GPS is operating and not that of GSM. You can even be tracked without mobile and using GPS. But forget that happening in Pakistan. At least not from the tracking company.
CDMA in this regard is the better technology if you talk about tracking and jamming mobile signals. Everyone can't jam it.