Can a GPS device be tracked if it is stolen?

A few weeks ago my roommate and I were robbed. They took some small electronics such as a GPS device (such as one that can be mounted on the windsheild of a car) and an Iphone.

The police didn't mention anything of it when making a report, but would it be possible to track the GPS unit if the thief powered it up and decided to drive around with it, perhaps if the police knew the serial number? It is a GPS tracking device after all!

The iphone was promptly turned off by the thief but would it be possible to do the same with it since it contains GPS capabilities?

This leads me to a third question, Could the police use a GPS device to track a stolen car, provided that the thief was stupid enough to use it in the stolen vehicle and the police could be provided with the serial number? My friends car was also recently stolen.

Should have boughten insurgence:/

No. The little GPS navigators with the built-in maps have no capability to transmit. GPS is a receiver only system. Sorry.

GPS trackers have a GPS receiver connected to another transmitting radio, like a cell phone or a walkie-talkie. The GPS receives the location from the GPS satellites, then tells the other radio what to transmit.

GPS - No, they have no ability to transmit their location.

iPhone - Yes using something like the where's my iPhone service.
However unless the police think that the person who stole it is likely to commit a lot more crimes or hurt someone they probably won't be interested, a stolen phone is fairly low on their priorities.