There is a mod and a video that's pretty easy to find with a quick search. There may be multiple ways of doing it but the only method I'm aware of consists of rubber bands around the shifter shaft to keep it in the 3/4 gate and make it rebound to center after the shift.
After saying all that though I'm pretty sure that mod will only work on the PC where you can change the mapping if the shifter positions. GT5 won't let you do that as far as I'm aware