A gift card covers only one user and pays for the main subscription. If you want to add profiles to your gift card account, you need to have a valid payment method saved and the profile costs with be charged to this payment method every 30 days.
If you already have a family account with several profiles and want to use a gift card now, then you can redeem the gift card on your account page under "Gift card". The price for the main subscription will then be deducted from your monthly cost and you only pay for the profiles.
It doesn't matter when you redeem your gift card by the way. The gift card period will always start after your last paid day. If the gift card has a different subscription type from what you usually have, the change in subscription will take affect when the gift card period starts.
Gift cards are only available for the following markets and have to be bought on the respective website: Austria, Belgium, Denmark, Finland, France, Germany, Italy, Netherlands, Norway, Poland, Spain, Sweden, Switzerland, United Kingdom
Gift cards are not transferable between markets.