You get 3 derivatives of the 6 series, the coupe (2 door), the convertible (2door) and the Gran Coupe (4door) otherwise referred to as the GC, are you not referring to the GC?
I haven't anything about a GT version, that's just in the 5 series.
Anyway, my aspiration is to get to the M6 GC one day, it's a very hot looking vehicle, now regarding resale, you will be better off buying one 2nd hand, new you will take a knock, as the demand for them isn't high. My friend bought a new M5 for R1200000, drove it for a year or so and did 17000km sold it for about R950000! If I had the extra money I would have bought it as he has taken the major knock on it.