The closest I've come to a gulag is reading Solzhenitsyn. I've not read The Gulag Archipelago, but I have read One Day in the Life of Ivan Denisovich and The First Circle. While Klein merely argues that Guantanamo is neither gulag nor POW camp, I think it can be fairly said that for the inmates Guantanamo is, in fact, worse than either.
The great sins of the gulag were its scope and its indefiniteness. In scope, there's no comparison. Stalin imprisoned millions in forced labor camps, while only hundreds are imprisoned worldwide as enemy combatants. In indefiniteness, the gulags far exceeded Western prisons. Stalin's whim determined who went to the gulags, how long they served, and how they were treated while serving their terms. Although Klein suggests that sentences in the gulags were certain, the reality for many inmates was different. Released on a Monday, there was no guarantee that you would not be rejailed on Tuesday.
Life inside the gulag must have been as brutal and harsh as a Siberian winter. On the other hand, the engineers in the First Circle and the craftsman in One Day maintained a certain dignity. They had jobs and something of a community. This is where I suspect the gulags and Gitmo diverge.
I don't know what it's like Guantanmo, any more than I really know what it was like Stalin's gulags. My impression from news reports is of detainees in small rooms for most of the day, with little contact with other detainees, with little or no contact with the outside world, subject to repeated, intense interrogations that at least some consider torture, with no plan to ever be released.
To answer, then, the right wing critics of AI's report: I agree, Guantanamo is not a gulag. In some ways, it's far the worse. Does the word really matter? Tom Tomorrow handles that question.