This is what I found out from a US Springhills on how to apply gift cards

1) Physical gift card is very easy. Just swipe it on their system terminal, like authorizing a CC. The system will show how much value in the card. All the FD needs to do is to enter the amount to be taken. Done. The system will give a screen on the deduction and balance remained - this is the ONLY time they could print out the information.
Once they leave this screen, the only way to see the balance remained, would be on the Folio.

2) eGift Card.
- FD goes to GF screen = Gift Card Fosse
- Use F1, F3 to process gift card
- Post gift card $ to deduct
- Screen will show the residual value of the gift card after deducting the $.
that is when my GM messed up. He mistook the residual value being the $ to apply to the Folio.
see this long post in another thread. https://www.flyertalk.com/forum/31738720-post63.html
- When everything looks correct - FINALIZE it.

I am not sure if once Finalize, to apply to Folio - would it be reversed. It does look to me it could because he inadvertently reversed the $50 Platinum gift Marriott sent in Spring - it was applied in a split way - one portion to Folio, one portion as a "Gift Card Sale" and added the amount to a physical gift card. Then during one of the attempts he tried to close the Folio, he reversed the $50 - back to the eGift Card. We have seen the $50 deducted screen in the Marriott Gift Card site in my account. I did not take a screen shot then. But when we returned to the account, we saw the $50 deduction was gone and the balance back to full $50.

So my guess is, as long as the Folio is NOT Closed, meaning, before the nightly finalizing bills, after auditing - any charges can still be reversed, from CC to Giftcard even Cash.

The stay has generated 4 folios with all sorts of payment method combinations shown, incl cash that has never been used by me. The GM explained that, after guest check out, the folio will go thru final auditing happened after midnight, usually between 2am to 4am. Then the true final bills are generated. After that point, it would be much more works involved to try to alter payment methods.