Points cannot be earned on orders that are fully paid for using a gift card.
If the total order value is covered entirely by a gift card (resulting in a £0 balance at checkout), no points will be awarded. This is because points can only be earned on orders with a payable balance.
However, if you use a gift card alongside another payment method (e.g. card or PayPal) and there is a remaining balance to pay, you will earn points on the amount paid (excluding the gift card value).