Back in GTS the problem was that it's possible to use both the H-shifter and the paddle shifters at the same time. That (and the fact that manually shifting with the H-shifter was faster than with the paddles, due to the very slow auto-clutch) was exploited.
Example: Imagine driving one of the slower cars at Tsukuba where only the second and third gears are needed. You'd leave the H-shifter stick in third, and while braking for a turn you'd use the paddles to shift down into second, then when accelerating you'd briefly engage the clutch and the car would change quickly into third. Then rinse and repeat for ~half a second advantage per lap.
I don't know if it's the same in GT7, but if it is, a smarter way (not a PD thing - I know) to stop the exploit would be to disable the game's automatic shifter mode switching - like in every other sim where you have to select gear switching mode before entering a race.