The fictional high-speed track is Route X, officially named Special Stage Route X. It is a vast oval-like test circuit built around two extremely long straights and steeply banked turns, making it ideal for testing acceleration, top speed and drafting.
Route X is often associated with Gran Turismo 6 because it appears prominently in that game, but it actually debuted earlier as downloadable content for Gran Turismo 5 in January 2012. It then returned in Gran Turismo 6 and later appeared in Gran Turismo Sport and Gran Turismo 7.
The circuit is fictional, although its concept resembles real-world proving grounds such as Volkswagen’s Ehra-Lessien facility. Its complete lap is about 30.3 kilometres, with the straights stretching for roughly 12 kilometres each. That makes it one of the longest and simplest tracks in the series: drivers spend most of the lap accelerating in a straight line before taking the banked corners.
Route X is commonly confused with older test courses or with Special Stage Route 7, another fictional high-speed circuit in the series.