If a guest with a hidden (or private) email account ("Hide My Email" setting from apple) tries to gift or receive a gift card, the transaction it's not going to be possible.
In order for the gift transaction to be successful, the transferee would need to have a valid guest loyalty account, which requires an valid email address, and on the other side, the transferrer of the gift card would need to provide the email address of the transferee in order to complete the transaction (per: How to transfer the gift card using the app from one account to another?)
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