I mean, I think Gitmo is monstrously unjust. But there are lots of injustices that are perfectly legal. I also think how the US treats its many many prisoners is monstrously unjust.
United Nations Convention Against Torture (torture itself, and rendition - refoulement)
International Covenant on Civil and Political Rights
Geneva convention
US Constitution - which is apparently now only applicable at the government's discretion
I'm afraid I don't subscribe to the sophistry which claims those detained are 'enemy combatants' to which no law applies simply because of a suspicion without charges or evidence, or that waterboarding is not torture. More important than the arguable illegality or (at best) extra-legality of the camp though, is the enduring damage it does to the reputation of the US throughout the world.
I mean, I think Gitmo is monstrously unjust. But there are lots of injustices that are perfectly legal. I also think how the US treats its many many prisoners is monstrously unjust.