Of course! Your right to return an item is exactly the same regardless of whether you bought it with a Gift Card, a payment card, or a combination of the two.
If you paid for your whole order with a Gift Card, then the refund will be credited to your ASOS account ready for you to spend straight away. You'll see the refunded amount at checkout when you place your next order.
If you paid with a combination of payment card and Gift Card, we'll refund anything you paid for with your debit/credit card first, and then the remaining amount will be credited to your ASOS account ready for you to spend straight away.