Are you talking about that hardcore of 13+ million?
I'm sorry but when people start saying that GT doesn't sell consoles they might as well stick carrots up their nose and run around doing a chicken impression because it makes about as much sense. GT3 sold millions of consoles. Sales of the PS2 had slowed right down until it was released and the PS2 & GT3 pack sold shitloads.
The first GT game on a PlayStation always sell far more than the second because the first one represents the huge graphical leap while the second is usually just a slightly upgraded version. GT5 will sell a huge amount, probably 10m +. Its the first GT on this generation and its the first GT with online play which is a massive addition.
Realistically, what percentage of the people that bought Gran Turismo will spend $400+ to play the next installment of Gran Turismo?
You have to remember that a lot of people buy games like Gran Turismo or even Mario Galaxy because they're high profile games on a system they own, not (necessarily) because they have any attachment to the series. This is how a game (like Mario Party 8) can be the weakest game of a series and still outsell every other game in the series by a wide margin, and a game can be the greatest game of a series and still be the worst selling game of the series.
People who are dedicated fans of games like Gran Turismo, Ratchet and Clank, and Everybody's Golf bought will buy into the PS3 based on the anticipation of these games.
This isn't saying that a game like Gran Turismo 5 will not sell really well, but it certainly will not come close to the sales of Gran Turismo 3 because there are too few gamers to buy the "high profile" racer on the PS3 and its sales will (mostly) be limited to Sony fans and PS3 owners.