According to the FAQ, Luxury Collection is supposed to accept gift cards.

https://gifts.marriott.com/faq.html

I had an instance at Delta Toronto Airport where they wanted to take my gift cards, but didn’t know how. Agent was convinced it would be processed as Cdn $ instead of USD (which I highly doubt). I just told them to not worry about it and paid with Amex. The next day I got an email from the GM apologizing and offering me 7500 Bonbon pts as compensation. OK, thanks!