In the EU a Gift card (single-purpose voucher) has to be sold with tax applied to it - in other words, gift cards are taxable products also before they are redeemed.
Example: A gift card with the face value of €100 including. tax, will be €80 excluding tax at a rate at 25%. The Gift Card has to be sold at €100 including. tax.
It is required by law in the EU to sell Gift Cards incl. tax and this can't be configured with the current configurations available in the Commerce Gift Card module.
Commerce Gift Card product type does not have an option for setting it to use a Tax Class:
(Not to be confused with “Tax Class for Gift Options” which is another functionality)