It seems Gran Turismo 6 is indeed in development at Polyphony Digital.
The L.M. Gianetti, which is a famous factory that produces race cars, uploaded a bunch of new photos on their Facebook Page, titled “SCANSIONE PER GRAN TURISMO PS4”
Update: The pics can be found here.
Translated into English it reads: “Scan for Gran Turismo PS4”
The game was recently rumoured to be released on the PS3, and this revelation means that it could probably be a cross gen title. GT5 has shipped 9 million units so far and the series is at 68 million units sold.
This is still unconfirmed at this point, but people have captured pics of Polyphony Digital scanning cars before, and considering the Facebook page is legitimate, Gran Turismo 6 may indeed be in works for the PS4 as well.