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 on your account page.
3. Activate your main subscription.
4. Add up to 4 profiles for family members.
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.