How can I unlock the Ford GT90 in Gran Turismo 2?
According to multiple websites, winning the Seattle Circuit 100 Miles Endurance Race will give a chance of unlocking the car. I can't confirm if this information is true, though.
Edit: Another website states that you need to unlock the Nissan R390 GT1 '97' Roadcar before winning the Seattle Circuit.
(Note: Most of these answers are user submitted answers on other websites.)