Yep, they will take your gcs early. If you're calling ahead of time, I *think* you have to do it 3 days in advance. Everything seems to be 3 days in advance of embarkation. Combine your gcs ahead of time up to $1k per card, and it'll make your life easier.
You can pay for your
DCL cruise booked through Costco with gcs. What they'll do is patch you through in a 3 way conference call with DCL, and you just give your gc info that way. The Costco agent I got even jotted down each gc # on his end, noted the amount on each card for reference, and even did the math all along the way, which turned out to be a good thing because the DCL agent was kind of frazzled, and got things mixed up at some point.
Edit: I had a similar issue with a port fees refund. I booked my placeholder with a gc, but I kept the gc, and I made notes and everything on it. When it came time to book the actual cruise, at the time, Disney Vacation Account was still running, so I used it to pay the rest of the deposit. Well, a few months after that, Disney decided to close the DVA and offered a bonus to those cashing out their accounts into an electronic gift card so I went for it. Fast forward to recently when the port fees had dropped. They refunded the amount back onto the gcs I'd used, so I went through and checked every gc I had literally six times. Maybe more. The refund wasn't on any of them. I called DCL to figure out what happened, and I guess they tried to refund it back onto the DVA, but since that account was closed, at no fault of mine, the money went into a limbo of sorts and no one could figure out where it went. Eventually, after a lot of circling around and around, they were able to figure out that it went onto the electronic gift card that was issued to me after the DVA closed. I didn't think to check that one because it wasn't in my stash of physical gift cards. I had it saved on my computer. Thank goodness. WHY DCL didn't put the refund back onto the last gcs I used, I don't know. Not only that, but DCL had spread the refund out...part of it was on the gc I used for the placeholder, and part of it was on the electronic gc from DVA. So yeah, save every gc unless you are SURE you're fully done with the trip you used it on and there's zero chance for refunds.