GTEs do rolling starts in real life and only have to accelerate out of pitstops with pit limier active.
Genereally 3/4ths throttle and feather in the clutch to avoid wheelspin. Clutch should be fully released you're around 60-70 kph - use as much throttle as possible, again avoiding wheelspin. This number will vary for different cars and track conditions.
It's probably something you'll need to practice honestly.