I do share the criticism of the American policy vis-a-vis detainees at Guantanamo - as well as those being held in Iraq and Afghanistan and I absolutely refuse to rationalise any acts of torture: they are - if you believe in human rights - always and everywhere unjustified and my own view is that Guantanamo should be closed as a matter of urgency.
However, SIAW is surely right to describe the comparison as completely lacking any sense of proportion - and Amnesty's use of it is part of a wider trend of trivialising historical horrors by applying them to contemporary cases that simply don't match the original events either quantitatively or qualitatively.
Practically any case in the world that involves widespread killing is today described by someone as "genocide", or a "holocaust"; any infraction of civil liberty, regardless of how trivial, is taken as proof of a "police state"; and everyone knows about the long and ignoble history on the left of using "fascist" or "Nazi" as epithets to discredit ideological opponents.
In the case of this use of gulag, quantitatively it is, of course, a wild exaggeration: the Gulags were set up in 1919 as a system of forced labour camps. By around 1934, these Gulags had several million members. The regime was brutal in the extreme with death from disease and exhaustion commonplace. There is no doubt in my mind that the inmates of Guantanamo Bay have had their human rights violated, which is disgraceful - but it is not a forced labour camp and there are around 500 inmates.
I'm aware of how tempting it is to use historical comparisons to win arguments; I confess - and this as someone who really should know better - that I've done it before and I think I may even have used this one before - but I'm going to make a special effort not to because it represents a very serious degradation of the historical language. This must be resisted because apart from anything else, it allows states to deflect perfectly valid criticisms of their conduct as wild and irrational.