Apple TV

Yes. Subscriptions will be automatically paid for from existing Gift card balance on the account.

Redeem the gift card onto your Apple ID: click here-> How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

Check your Apple ID balance: click here-> Check your Apple ID balance - Apple Support

Yes. When you redeem the gift card your account will be credited with that amount and it will be used before any other payment method associated with the account.