This could be a logical start to discount reductions. After June they could then reduce the $ amount over time until it disappears altogether.
Or perhaps there will be strings attached. For instance, what if they give you the gift card but it is only valid after that trip (gets mailed to you after the trip), or maybe it's only valid toward the purchase of a future trip, and maybe with an expiration date that would force you to use it or lose it. This would entice people to return again in the future. Plus, if something like that was done, not all of the cards would get redeemed, meaning that they wouldn't have to shell out all of the money. Look at the millions of dollars in gift cards that never get redeemed in this country. Gift cards are a money maker for stores since so many cards never get used. (It wouldn't be a money maker in this case, but the full cost to them wouldn't be realized, compared to free hotel nights which cost them the full loss of room revenue on every single vacation booked. With my scenario not every vacation's gift card would be redeemed, meaning they wouldn't lose money on every vacation booked with the promotion)
I don't see them doing it without strings attached, and more than just a minimum stay requirement.