That's the RA6 Woodward concept GTO. No spoiler exactly like that exists as it was entirely custom.
Closest thing available to my knowledge was the DMS Duckbill Spoiler which has been out of production for many years.
2005 M6 GTO
CGM | CCW SP550 | Pedders Xa | Kooks | Spintech | Ed Curtis Cam | FAST 92 | Duspeed | GMM | Street Slayer Clutch | Billet Prototypes | JHP Pod, Spoiler, Steering Wheel | GTOG8TA SAP Grilles | Etc.