A notable "feature" (actually a hidden cheat discovered years after release) allows you to instantly unlock every car in the game. To trigger it: Navigate to the main menu. Start + Select + R While holding those, input the following sequence:
Create a folder with the game's ID (e.g., SCUS94423 ) inside PSP/GAME/ on your memory stick. Place the EBOOT.PBP file inside that folder. gran turismo psp eboot