Gitmo would be considered a treaty like the Panama Canal giveaway. It would require a 2/3rds approval of the U.S. Senate. At least least it would have back in the days when rules meant something.
With what we have in the Senate these days, getting 2/3rds approval will be no problem. Mark my words, the closer Obama gets to leaving office, the worse it going to get and there isn’t anyone in congress who will lift a finger to stop him.