From GTPlanet Game Guides

High Speed Ring

High Speed Ring is a Fictional track that has appeared in Gran Turismo 1, Gran Turismo 2, Gran Turismo 4, and Gran Turismo 5: Prologue, the track did not appear in Gran Turismo 3: A-Spec, due to Namco claiming that the track was based off of the track in their game "Pole Position," without Polyphony Digital obtaining the rights.

The track consists of 7 turns, followed by a long straightaway. As the name suggests, many of the turns in the track can be taken at a high speed, and it was one of the few tracks in previous Gran Turismo games (until the inclusion of Daytona International Superspeedway,) where you could run at or near full throttle for most of a lap.