I recently cancellled a stay for tonight that would have triggered the credit. I needed to act quickly to retrieve credit before anniversary date, so I purchased a $300 gift card online at Marriott’s website. Purchased 12/22, posted 12/24, credit posted 12/26. I was monitoring just in case the amount of the gift card would somehow not trigger credit (I.e. Delta GC must be no more than $50 for airline incidental reimbursement) on other Amex cards. So, I wanted to share it DOES work.