I very strongly suspect you would not be allowed to use the GUC, though I can't find anything in the T&Cs that clearly prohibits it. If the flight originates in the US I suppose you could buy the ticket and call Delta right away, and if they can't do it, take advantage of the 24-hour cancel policy. (Note that you do have to call Air France to use the 24-hour cancel though, it can't be done online.)