Unless he challenges the legality of the court itself. An illegal court can't render a legal conviction, no matter what process it pretends to have followed. Whether he will do that I don't know--he might be satisfied to sue the Canadian government for allowing him to be tried by an illegal court.
I can't believe you have such a hard time grasping that when faced with an unfair, biased, and crooked court someone who enters a plea deal is actually being extorted. I don't see his plea deal as being in ANY way voluntary, given the unfair, biased, and unethical process he faced at Gitmo.