To our knowledge this game does not exist but I know it does. Yes what Amar said made is terribly obvious that a GT PSP game is in production. With that said when (if) GT portable is announced for PSP how many copies do you think it would sell?
My guess would be 2m lifetime WW due to the franchise being the biggest Sony has and software sales for PSP games outside Japan being pathetic.
Edit - My retardation is becoming acute now, the title should have been "Predict GT portable PSP sames"