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. These will be charged for monthly however.
Here's what you need to do:
1. Redeem your gift card.
2. Add a payment method in the "Payment information" section under "My account" on your account page.
3. Activate your main subscription. There won't be any activation charge as you already have an active gift card.
4. Go to the "Profiles" section on your account page and add up to 4 profiles for family members.
When add the profile, you will pay for the days left in the current period. Then, the payment for the profiles will be charged every 30 days until the gift card time has run out. After that you will be charged for your main subscription and the profiles together each month until you cancel either the profiles or your whole subscription.