A GPV (Gravity Powered Vehicle) in this instance it is in the form of a bike. These bikes have many designs and come in many different forms but proper ones follow these guidelines. No driving chain, cranks, sprockets, or chain rings. These bikes are solely meant for going downhill as fast as possible. The idea first arose in the early 80's in California back then it was something to do for fun guys would build their own gravity bikes from any random bike parts that they could get their hands on trying to build the fastest racer for the cheapest price typically from old BMX frames. There were many guys that would show up to these unofficial events and race what they had, basically there were no rules except race what you brought. Some had fairings, some had added lead to random parts for the extra weight. Eventually the cops found out and tried to stop it giving tickets to riders from impeding traffic for going to slow, speeding if going to fast, and no headlights for night runs. Eventually the gravity bike craze mostly died out. . . or did it?
In the late 80's BMXplus had an article about the craze which inspired many BMX riders from all over the world to ride. One rider (Jared Hanebrink) was featured in both the BMX Plus and later the Action Now magazines for GPV riding with his full fared gravity bike (No. 1, photos below). Dan Hanebrink became a big name for gravity bikes pretty soon after the beginning eventually riding his fully fared gravity bike in the 2000 X-Games when it was featured as a demonstration sport.
Photo's taken from an old issue of "Action Now" Magazine