Customers who have a gift card can redeem their code via a special redemption page.
Clicking on the Join Page or Get Access Now button will allow your customers to claim a gift card.
This is a 2 step process.
Redeem the code and create an account as demonstrated in this gif below.
The redeemed gift card amount will be added to the customers balance.
While logged in, they can go back to the join page or click on content in the catalog (via the 'Get Access Now' button), and select a subscription option. During the checkout, the end-user's gift balance will be applied towards their chosen subscription.
A credit card is only required to enroll in a subscription if the gift card balance is less that cost of the first subscription charge.