From what the analysts at Cowen & Co are estimating, Gran Turismo 6 which was only a PlayStation 3 title has only been able to muster about 300,000 in sales compared to Gran Turismo 5 which sold 1,169,074 copies in the same time period last time around. Sony has not commented on or offered any sales figures Gran Turismo 6.
The decision to go ahead and release the 6th installment for the PlayStation 3 might in the end be viewed as a very bad decision by Sony. It should have instead gone head to head by bringing the 6th installment out for the PS4 and by doing so go head to head with Forza 5 on the Xbox One.
It does seem logical now that Sony will be pushing hard to get a new Gran Turismo release for the PlayStation 4 sooner, rather than later.