I work for a company that has a software solution for many types of devices (he have more than 25.000 cars monitored in Europe). I am currently working on that project.
While you may find GPS devices you also need to have access to a software solution in order to see where the car is. You also need to configure the GPS device to your exact needs (so that you don't have to much or to little data, or have battery drainage issues).
Yes, all GPS devices (including the navigation systems) can be jammed with a device. It just cuts the GPS signal on a 5m(ish) range. But, both the device and it's antenna are hidden from sight and are situated in the dashboard. So, you can only see that a car has a GPS tracking device if you take the dash apart and know where to look. Plus, the device that jammed the GPS Signal has its power output from the lighter and the GPS tracking device has it's power directly from the battery. So, when the thief takes the car to a specific location it might send data after the car is stopped and the GPS jammer is no longer working.
In addition to this, you can set the GPS tracker to send data after a period of time. For example you can set the GPS tracker to send data at every hour (or even less depending on the battery size and usage) and get a SMS or email or both, when it is not working. So the thief only has an hour head start.
In Romania we have used this device for anti theft and is worked very well. For example a had a colleague who's Audi A6 was stolen during the night and, the next morning, he accessed the web application, saw where his car was (250 miles away), called the Police and they when to that specific address and caught the thief.
For this example the thiefs replaced the ECU so a remap and SPS Anti Theft mode only works with the original ECU.
As I said, we use a lot of different device types but the most common device is Teltonika FM1100:
http://www.teltonika.lt/en/pages/view/?id=1021Hope this helps!
Edit: I forgot... When you lose the GPS signal you can be notified via SMS or Email in real time.