Help? Transmission Tuning for Grid Starts?

United States
Nor Cal
I do GR3 racing almost exclusively and 99% of the time I use a rolling start, or the GTSport mode. Due to this, I've always used my 1st gear tuning to maximize the lowest speed corner on the track.
Recently I've been racing with new people who use the grid-start option, and my transmissions are garbage at launching. I lose 5-8 spots off the line, which in a 5 lap race, when accounting for trying to make it through traffic, is basically sacrificing any chance of winning. This weekend I've been experimenting with TCS, gear ratios, but am having zero luck. I have a points/team race in a few hours, and I'm hoping someone can give me a head start, or breakdown of what works for them, or success/systems you've used in the past to best get a good launch off the line in GR3 cars.

If it matters, I use manual, T300, and the car changes based on track, Beetle, FT1, Puegot, etc.
Thanks in advance.