Combine Apple Gift Card and credit/debit for purchase?

1. When you say ‘Store’, do you mean the brick-and-mortar (and often much glass) Apple Store, or the online Store?
2. Gift Card use may differ by country or country-like region. So for which country?

Gift Cards would first be redeemed onto your Apple Account, as account balance, and then be used for products (or services).

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

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

In general, when buying a product (or service) using account balance from Gift Cards, any unpaid remainder can be paid using a secondary payment method, like a credit or debit card.

FWIW, these Apple Community forums are peer-to-peer support forums. You are not speaking to Apple here, only other users, and we understand Apple don’t actively participate here.

I don't know quite the appropriate contact point for your question, by you can start here with a phone call:

Customer Support (800) MY–APPLE (800–692–7753) or on line /getsupport.apple.com/