Identify who owned the balance before shopping
A gift addressed to the household can pay a household cost. A personal gift card still belongs to its recipient unless they choose to contribute it. The same shop receipt can look identical under either arrangement, so ask what the owner intended before calculating anyone’s share.
Separate the game's price from the extra payment
When the household owns the card, mark “price $28; household gift card $20; Chris paid $8.” Chris and the other adult each cover $4 of the extra cost, leaving the other adult to reimburse Chris $4. Do not ask the other adult for $14 as though Chris supplied all $28.
Keep track of remaining credit and returns
If the gift card belonged to one person, ask whether they intended to contribute it as a gift or receive credit for using it. Agree on that before recording a balance. Keep the card until you know how a return would be issued.
If the $20 card was personal and Chris wants its full value counted as a contribution, Chris supplied $28 and the other person owes $14 under an equal split. Keep this decision with the receipt. If a return later puts credit back on the card, you will know who should benefit from it.