Sony has announced that Gran Turismo 5 has managed to ship over six million copies worldwide since its release in November 2010 - more than 50 percent of which were in the Europe region.
This follows the release of the v1.06 patch, which enabled new things like, the ability to access B-Spec Mode from a web browser, as well as an improved online system.
It's worth noting that "shipped" doesn't mean "sold". It's a term which Sony use to denote sales to retailers, not sales to consumers. The lowest selling "shipped" game, aside from Gran Turismo 5, is Gran Turismo 2. That title managed to ship 9.37 million copies.