Is that true? I've played GT for ages and I've never been able to do a proper start that ended up being faster than mashing the throttle at the start.
well... yes and no... you can configure your clutch to launch into second, and you'll need to shift manually... but that's generally not a good idea for actual racing, more so if you're drag racing, but regular racing, that's not particularly advisable... so while it's possible, it's not really recommended, I've never done this in GT... plus, you can't tune license cars, so it's sort of pointless...
but yea... it's better if you time your launch... it depends on the car where that sweet spot is but generally, you want to launch somewhere before the red... 4000-6000 rpm or so... I would suggest trying to get your rpms at the start about 3/4 of the way to red... this isn't precise, but you can experiment... regardless, even if you sort of miss, it'll still be better than holding down gas before the race starts...