First off, get a Le Mans car. You can find one at the online dealers and a few more at the Premium section. Choose wisely and then tune it to the max. You should also include a Racing Soft Tire for your Le Mans car (You might be able to win if you equip racing soft tires with your Zonda R/Mazda Furai, it makes things a lot easier).
Once getting a fully tuned up Le Mans car, the first four Gran Turismo All-Stars races should be cakes, but be careful with Tokyo R246, which I consider to be one of the hardest tracks of the game. You need to overtake as many cars as you can and don't make mistakes, otherwise, you will lose the race.