How do I redeem a Gift Card code in my game?

You have to redeem the Gift Card first, there is no option to enter a Gift Card code in a game to redeem.

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

Is the amount of the Gift Card reflected in your Apple Account balance?

Check your Apple Account balance - Apple Support

If the amount is showing there, then it may be the case that you still have to have a valid credit card added. The purchase would still be made from your Apple Account balance and would not charge your credit card as long as funds are available in your Apple Account. The credit card region must match the region of your Apple Account, so it is a way to verify that you are using the correct App Store. If you would like to speak to Apple where they may have more information about your account than what we would know, you can do that here:

Contact Apple Support - Apple Support

Then contact apple support or the developer of the game. This is a user forum. You are not talking to Apple.

Danialhrkd wrote:

I need an online chat with support to solve it

If online chat is available in your region, then you can access it with the link I provided to contact Apple directly.

That's nice for you...

We can't do anything about it. We are NOT Apple.