GT by Citroën is a European exclusive Special Event Blu-ray Disc (BCED-00393) that was not available for purchase. It's discs origins are currently unknown but was likely shown at the 2008 Paris Motor Show with the debuting GT by Citroën concept car as part of a collaboration between Citroën and Polyphony Digital.
The GT by Citroën was included in Gran Turismo 5 Prologue as part of a downloadable content update the following day on 3rd October 2008. It features realistic driving dynamics and is available for players to race in various events within the game.