is the education store gift card free

No. Any Apple Gift Card can be redeemed against a purchase without incurring any additional costs. The full balance is available to spend on Apple goods or services.

How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

Free items from Apple are technically free, but Apple charges the full price on the free item and discounts the primary item that triggered the free item by the amount of the free item.

For example, if the offer is for a free $100 gift card with the purchase of a macOS device, then the gift card will show $100 as the charge on the receipt, but the macOS device will show $100 less than the listed price.