It's unfortunate that the term "Gurney flap" and "Gurney" are being incorrectly applied to a simple rear spoiler design on the M2 CS. I don't understand where this is coming from but apparently BMW themselves?
In order for a "Gurney flap" to exist, you first need an airfoil, a wing, with clear flow on the top and bottom. Next you need an airfoil shape to said wing, and in the case of a car, the Gurney flap is pointed up at the rear edge of the airfoil. It increases pressure on the high pressure side of the wing while at the same time helping the boundary layer to stay attached on the low pressure side at greater angles of attack.
No such wing exists on the M2 CS. It's just a larger rear spoiler with a reduced profile in the center. It has no relation to a true airfoil with clear airflow on both sides where a real Gurney flap could be useful (increase coefficient of lift (i.e. inverted on a car wing) without changing the airfoil camber.
/rant off
