Mazdas
There are two types of Mazdas you can unlock. To unlock the Mazda RX-7 tune, get a 20+ max combo on the Heights Mountain leve. To unlock the Mazda RX-8, win the Championship Race on the beginner level.
Nissans
There are three Nissan cars that you can unlock. To unlock the Nissan Silvia S14 Tune, defeat the first level in drift mode. You can unlock the Nissan Silvia tune (S15) by getting a 15+ max combo on Downtown Street. Unlock the Nissan 180SX S13 by defeating the Enjoy Cup on Beginner level.
Toyota
There is one Toyota players can unlock. To unlock the Toyota Celica LB TA27, win the 3 Door Cup on Beginner level.
Levels
By defeating one level, you can unlock more advanced levels. Defeat the beginner level to unlock the intermediate level. Defeat the intermediate level to unlock the highspeed level; and defeat the highspeed level to unlock the professional level.
Miscellaneous
There are a few other unlockables in "GT Pro Series" as well. Defeat the Championship Race on Beginner mode to unlock Brake Street. Defeat the Enjoy Cup on beginner mode to unlock the Filter Grade-A and defeat the 3 Door Cup on beginner mode to unlock headlight blue.