It's ok, thanks for reply guys, selected auto 3D mode outside the game. But tbh I'm not overly impressed with the 3D mode, to me the racer/car text stands out a bit but that's about it. I really hope bluray 3D movies look better.
You will need to change the settings in GT5's options menu for it to work properly, scroll to the right in the hardware section and select 'On' in the 3D settings. The TV should automatically change to 3D mode.
As jakhole said, you will want to play with the settings so it's most comfortable. I tend to use parallax:8 and convergence:0.75, but it varies by TV, eyewear and distance from screen