You will have to read everything carefully.
"As a reward for playing the heart racing GT Academy 2013 demo, you’ve been given a free in-game Nissan GT-R GT3 for Gran Turismo®6. So when the stunning PlayStation®3 driving game launches on 6 December 2013 you can get on the starting grid in style.*
*Once you have your copy of Gran Turismo 6, you will receive your free in-game Nissan GT-R GT3
"
I have played, and GOLD every event in the game. I noticed other Portuguese players claim to have received this email, but I have not.
Will keep checking my inbox more frequently.
However, I don't believe the email matters that much. If you played the demo, your account has automatically registered. Reason why the rewards were sent out in GT5.
I believe everybody who played the demo will get the car once they start the game.