To explain gear ratios: Basically you need to adjust ALL the gear ratios slightly until you get a basic set of ratios that make the gear shifts sound normal, and not too short or too long.
Then, if the track is Monza for example, adjust the 3 top gears so you get your top speed in top gear down the longest straight, with 3 or 4 of the LED revs lights lit up. Make the top gear slightly longer so you can have a bit of room for overtaking because your revs will need to be able to go higher when slipstreaming another car. (If you are hitting the rev limiter all the time even when not overtaking, you will have no margin to increase revs when you need them to overtake, so top gear need to be a bit longer)
Then if the gear shift sounds quite normal through all the gears, you have a pretty good setup. If one particular gear sounds a bit long or short, adjust it so it sounds right.
I never understood gear ratios when I started playing GP3, but it is important to change ALL the gear ratios, so you don't get a funny sounding engine. Never change only 1 or 2 of the gears, you need to change all of them to get the best.
For Monaco on the other hand, you would generally shorten all the gears, so that you were hitting the top speed in top gear on the longest straight.
Hope this helps you Marcow_nl