We are at WDW right now.
I called the day before our arrival date and it was no problem for the CM to apply the Canadian gift cards to our reservation over the phone.
My daughter used the Canadian gift cards at the resort for her room and as others have said, the first CM they dealt with told her it wouldn’t work and told them to call the number on the back. The CM at gift card services told her that (as the person above mentioned), they must be entered manually like an e-gift card.
Then they had to get back in line and once she told the second CM how it had to be processed, it was no problem.