GPS technology for collars typically sends a text or email notificaiton when your pet leaves home and gives you the ability to track your dog on a map using a computer or cell phone.
Having a GPS unit on your dog's collar seems a bit excessive and it could give people a false sense of security. A GPS unit isn't going to stop your pet from running in front of a car or eating something toxic. However, if one of my pets ever got lost, being able to track their exact location would be invaluable.
GPS units for dogs are surprisingly small, but they're still a bit bulky to have on my dog all of the time. For now I think I'll stick to the good old metal identification tag and microchip.