Gran Turismo Hints 

                                         

All 8 tracks:
Go to Arcade Mode and enter the "Bonus Items" option. You should see the
letters A, B, and C above each of the levels given. As you do the following,
they will be covered with a blue, yellow, or red dot. To fill in the four
big question marks, go back to the main Arcade Mode screen. Then, go to
"Single Race", choose any difficulty level, and place first in each of the
four original levels with an A class car (ex: Mitsubishi GTO), a B class 
car (ex: Mitsubishi Eclipse), and a C class car (ex: Honda Civic 3-Door).
After you have done this, go back to the "Bonus Items" menu and the four
big question marks should be filled in with these levels: Autumn Ring,
Deep Forest, SS R5, and Grand Valley Speedway. That makes 8 tracks!

New Dealers:
Beat all 8 tracks under 'Easy' and get 4 new car groups - Toyota, Subaru, The New Dodge, and TVR.

Ending Movie:
Beat all 8 tracks under "Normal" and get an ending movie (credits).

Hi-Res Mode:
Beat all 8 tracks under "Difficult." Hi-Res mode makes the graphics less pixely and makes the game have better sound.

Chrysler Concept Car:
Get all gold medals in the B Class license test and you'll get the Copperhead. You can also get it by getting first in the UK vs.
US championship. 

Toyota TRD3000GT: 
Get all gold medals in the A Class license test.

Nissan Nismo 400[R]: 
Get all gold medals in the International A license test.