You can only pay for your main subscription with a gift card but if you add a valid payment method, it's no problem to add up to 4 profiles. Your extra profiles will then be charged to your payment method every 30 days while your main subscription keeps being paid by your gift card.
Here's what you need to do:
1. Redeem your gift card. Go here if you need to know how to redeem a gift card.
2. Add a payment method in the "Payment" section under "Account Information" on your account page if you haven't got a valid payment method saved yet.
3. Activate your main subscription in the "Subscription" section on the same page. There won't be any activation charge as you already have an active gift card.
4. Go to the "Profiles" section in the menu and add up to 4 profiles for family members.
When adding the profile, you will pay for the days left in the current gift card period. After that, the payment for the profiles will be charged every 30 days on the same day your gift card period renews until your gift card has run out. At the end of your gift card periods, you will be charged for your main subscription and the profiles at the same time each month until you cancel either the profiles or your whole subscription.