If you want to try to have the best of both worlds I have come up with a good general strategy that has worked spectacular in reducing my lap times. Instead of just tinkering with the overall gear ratio what you need to do is set the individual gears up right.
The current set up that I have been experimenting with is to raise my overall gear ratio to almost redline at the end of the longest straight using the automatic gear ratio selector. You don't want the car to be at the top of the revs, because almost no engine makes its max power rating at redline. Then you will have to go into the individual gears.
Adjust 6th gear as short as it can go, as well as 5th gear. Then adjust 1st and 2nd gear as long as they can go. Observe the graph and adjust 4th just before the middle of 3rd and 5th gear. DO NOT touch 3rd gear.
The reasoning for this setup is that there really are no first gear corners in Gran Turismo (there are some, yes). Then I raised second gear because most of time that is your lowest driving gear. Raising second gear all the way DOES NOT make it to long, instead I use it as a way to control tire spin coming out of second gear corners ( I don't drive with electronic nanies on ). This increases acceleration and lowers your lap times. Now the rest of the gears will be shorter and always in the engines powerband no matter what corner your in, while still maintaining the top speed that you need and improving your acceleration too.
TRY IT!
( This is of course a generalization. After I do those steps somtimes I make MINOR adjustments to the gears or final drive. )