There is a full description here
https://www.gtplanet.net/forum/threads/praiano-gt6-tunes-gran-turismo-red-bull-x2014-junior.291084/ Its the second post.
In reality I don't really think you need to flip it you can just do the following.
1) Set the speed to cover the range you roughly want.
2) Adjust first gear to what you want, wheel spin is probably the main factor here make it taller for an easier drive.
3) Adjust top speed to what you want, Fuji is a good benchmark for your max speed needed, just turn around & do a run down the main straight. Sarthe & Route X are about all that's faster.
4) Now adjust the gears between to what you want, can do visually but the best thing is set yourself up a spread sheet or use the one on the link
If you don't flip the gearbox & use the spread sheet provided
For 1st & top enter the ratio you have decided on & set % to 0 for first & 100 for top.
For the other gears enter the adjustable range minimum & maximum.
Adjust the percentage to what you want & adjust the gears to the figure it gives you.
