Hi, I race in the xbox league, and in doing so have learned the importance of setup, and how much of a difference gearing makes.
First of all, you need to know if you want long or short gearing. Short gearing is good to get up to speed quicker, you will notice the difference in acceleration, however, top speed will be reduced.
An example of a circuits where short gearing is essential is Monaco and Hungary and Melbourne
For the circuits with longer straights and faster sweeping turns, such as Spa, Silverstone and Malaysia you'll want higher gearing. It will take slightly longer to get up to top speed, but speed is everything on the faster circuits, not acceleration, as you will be in higher gears coming out of most turns anyway.
Most circuits though are now a mix of straights and tight hairpins and chicanes, with some but not as many sweeping, fast turns.
So it then comes down to finding a gearing pattern you think works for you around a particular circuit and then adjusting to suit anything specific you might need.
For example:
At China, I have long gearing, for the long back straight in the DRS. However, my 4th gear is dropped 4 clicks lower, because of the way I take Turn 8. If my 4th gear was higher, I'd have to drop to 3rd to be comfortable taking the turn in my normal way, but by dropping the 4th gear a bit lower, I don't have to do that.
Another example would be Suzuka. I have a very long 4th gear. This is because I go from T2 to Dunlop in 4th. Making it longer gives me a bit more freedom on the throttle without worrying about high revs for too long (engine overheat, fuel consumption issue)
Hope that helps mate. Anything else just ask