Every human being deserves due process.
Says who?
Due process is an invention of civilisation, it is not available in nature (the tiger will eat you) nor is it a good deal or fair towards the uninvolved.
Within a legal system, like under a king or in a republic, due process is a good deal to keep the balance between government power and the right of the individual. But that is not the situation Guantanamo is in.
All wars see the innocent become casualties. But here's the thing:
Guantanamo keeps lots of terrorists out of the war, reducing the number of battles.
The innocents in Guantanamo (and I doubt they are a large percentage) are in the mean time enjoying a far better fate than the innocent casualties of the battles would have.
Sometimes the question is not right or wrong, but pure mathematics. Should we keep innocents alive and in prison or should we risk them (more likely others) being killed in battles that can be avoided by keeping terrorists in Guantanamo?
If the US would apply the idea that no innocents' rights must be harmed no matter what to the war (or any war), the US wouldn't be able to fight.
Tell that to the terrorists' victims in the west and the middle east.