I watched it through when it was up yesterday, here's how I think you do it (my prior knowledge combined with what I remember from the video):
1. Start up PCSX2 and GT3. Start up Cheat Engine and select "PCSX2" as the process.
2. Replace the string of the class you're replacing with "CLASS_PREVIEW". Upon selecting the class you replaced, you should be granted a selection of every car in the game.
3. Replace each opponent car string with the car you're driving. Then when you're all done, you'll have your own custom grid!
Did I get it right? I'm actually interested in trying this myself.