So I called Delta and the rep mentioned that in this case, they would waive any change fees and issue a credit of the full value of the ticket good for up to a year from the original travel date. As far as Chase Sapphire trip protection, I'm not quite sure what would happen but it seems like since they're 'detached' Chase would just credit me back the amount I charged on the card. I just realized I should have asked if they could issue a non-expiring credit only for the portion of the ticket I paid for with the GC... from the sounds of it though, they probably wouldn't do that.