I think you're referring to Launch Control, not Competition Mode.
Comp Mode is entered while in Sport or Track by pressing the TCS button twice. Launch Control is then available to you by holding the clutch in, car in 1st gear and rapid full accelerator application. When you release the clutch, LC will handle your acceleration through 1st gear.
Page 224-225
https://my.chevrolet.com/content/dam...ro1stPrint.pdf