Hey Red Rock,
Payment gateways aren’t that motivated to build $0 checkout capabilities because that transaction is worth nothing to them. It’s also s weird edge case since payment isn’t actually needed.
If you’re not taking payment, what is it that you’re trying to accomplish with a check-out process?
My guess is you’d be better off with an “email me this” form and a bit of automation to deliver the product- or perhaps using Memberships, and content gating.