After you purchase your first car in Gran Turismo 5 on the PlayStation 3, you'll unlock the National B License Tests. The first one (B-1) is a stopping challenge, and if you want to learn how to get the gold trophy, this video gives a perfect example of how to get it.
B-1: 200m Stopping Challenge
You may want to use the X and Square buttons instead of the right stick to accelerate and brake so that you know you're getting 100%.
It'll take a few tries, but try to find a marking on the pavement to use as a reference to start braking. If you end up running far into the target stopping zone, then move your braking point a bit closer to the starting line and try again.
Note * Played with a regular six-axis PS3 controller.