Do you use a controller or wheel? I tried the FGT a few times before I got my wheel and didn't stand a chance so I just gave up. With a wheel it's many times easier. A few weeks after I got mine I decided to get it over with and after a few days practise and some tuning I managed to win all the races except Monaco without aids.
I honestly don't remember how much of these settings are from someone else's tune and what I did myself, but the most important thing for me was to get rid of the lift-off and power over-steer which is why I have such high LSD decel and also less than max front down-force.
Race Medium tires
Aero: 65f 90r
RH: +5f -5r
SR: 18.0f 18.0r
DE: 8f 8r
DC: 7f 7r
ARB: 5f 6r
Cam: 3.0f 1.0r
Toe: 0f 0.10r
LSD: 10/15/40
Brakes: 7f 4r
Transmission set for each track by adjusting "max speed"
As for the actual driving... Run manual gears, stay in higher gears through low speed parts and rev all the way to the limiter when accelerating. Always try to straighten out the front wheels when coming down on the throttle, even if it looks like it will take you straight off the track. Once the car stabilizes and you get some down-force working you can turn harder again without spinning and hopefully still have some road left. I did a LOT of engine-braking/coasting through medium speed corners and S-curves without touching the brake pedal. Then just practise, practise, practise.
One thing to keep in mind is that it's much more important to be consistent than to be super fast, because of the AI rubber-banding when you're in the lead. 1:17's are enough to get the lead on Fuji but even if you run 1:13's you won't put any distance on the AI cause they'll just start running 1:14's... Because of that I ran with (I think) TCS:5 when I did the entire championships and went veeeery easy on tricky parts of the different tracks.
Also as Adrenaline said, play dirty! As soon as I got the lead in each race I made sure to block out the 2nd place guy in the standings just to make sure he wouldn't win in case I messed up.
