This and the oldest information on your account. For example the very first email address, phone number registered on the account or payment method.

If your account has been hacked before and then you add the gift card after the fact (after you got your account back), it's not going to be that reliable proof for support. Steam account ownership always goes to the account creator. There are multiple users who got denied access after getting hacked because the info provided is too recent. So best possible way to prove ownership is providing the oldest info possible