Per Title.
Gift card ids can be retrieved from REST API via order.transactions.receipt.
But I am unable to retrieve gift card ids used in a purchase (i.e. order, transaction) from the GraphQL API.
receipt is available through the Gql API but not documented. see https://community.shopify.com/c/shopify-apis-and-sdks/properties-missing-from-transaction-graphql/m-p/667313