If you return goods that have been purchased with a gift card, the credit will be added to your customer account and you will be able to use it for future orders.
To view your current gift card balance, go to 'My Account' in the top right corner then 'My overview'. Your current balance is then displayed in the 'My Vouchers' section.
If you have used a gift card to purchase part of your order and then you make a return of some items, then the gift card will be applied to the part of the order that you keep.
If you used a voucher code then the terms and conditions of the voucher will apply.