The previous Gran Turismo iteration sold around 10.5 million lifetime. Gran Turismo 4 was released on February 22th, 2005, while the userbase of the PS2 was about nearly 85 million according to
http://vgchartz.com/hwcomps.php?cons1=PS2®1=All&cons2=GC®2=All&cons3=XB®3=All&start=38039&end=38417&weekly=1
If the tendency keeps up, and considering this is the best-selling franchise for Sony... just take a look at GT5 Prologue...
http://www.vgchartz.com/games/game.php?id=6972®ion=All
You can see there's a keen interest in Europe. Considering the small userbase the PS3 have (likely 34 million when GT5 will launch) versus the 85 million PS2 when GT4 launched, I think, more likely, that it will sell more in Europe, and LTD about 7 million GT5.
3,5 millions Europe
800 000 Japan
2,7 million Americas