No, not necessarily. An on-board computer is not the same as GPS. It is the software that indicates how much gasoline you have consumed, how many kilometers you can still drive with your current fuel level, etc.
If you want to know if a car has GPS, you can see it on the car's detail page under vehicle equipment. If the car does not have GPS, but you want one, we can also install it for you.
You can also turn on a filter in the car overview to show only the cars with GPS. You'll find that filter in the "specifications" section. Or view all available cars with GPS through this link.