As of API version 2026-01, the Storefront API returns a GIFT_CARD_RECIPIENT_INVALID error when adding a gift card to a cart with missing or invalid recipient details, such as in this example.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/storefront-api-now-returns-errors-when-adding-a-gift-card-to-a-cart-with-missing-or-invalid-recipient-details