I have an used Celebrity Gift Card for $ 100. I chose this one from a work Christmas gift website (Giftogram) because I thought it would be a nice idea for my wife before the cruise. Ran into the same issue that I couldn't use it for OBC adn the cruise was already paid for. It was only new bookings only.
Happy to sell it for $ 50 to someone. Gift Card ID is unused. LMK if anyone is interested.