No, you can apply ahead of time using the QR code supplied by the dealer (just ask them - the QR code is specific to the dealer - so they need to send it to you).
You will apply via the application form, and if approved, get a temporary GM Rewards card number + expiry date + CVV. You must save these as that is what the dealer will use to charge $1000 for part of the down-payment on the car.
Note: you cannot use that temporary card number for anything other than a purchase at a GM dealer, so don’t go around racking up purchases on it until your official card arrives in about 10 days.