Your order confirmation email ("Thanks for your order") is your receipt, and it includes the VAT details.
If you have an account, your orders are also listed in your dashboard.
Gift cards themselves don't usually carry VAT when you buy them. VAT is normally charged by the retailer when you spend the gift card on goods or services, and they issue the VAT invoice for that purchase.
Buying for a business and need an invoice? See Business and bulk orders.