I concur with the objection raised by @albert: you can buy gift cards and print them out if desired. And the raw gift cards are subject to the usual stock and supply management.
If https://discuss.tryton.org/t/new-invoicing-method-when-selling-both-goods-and-services/345/5 is deemed to be needed, then well, it should be implemented and used. But the shortcut “because they are not stocked” doesn’t meet existing real use cases.