Another idea -
For those of you interested in trying to free Gyan Thunderheart, you could always just attack him. Re-deadification is the first step in de-zombification. If he remains undead, he's no worse off than he was before. If he really dies and his soul is released, then he's in dwarf heaven, and there's no one to take the sword north, based on the deal as it stands at the moment. Something would have to change, although there's no telling what that change might entail...
Of course, all of that assumes that you have the capability of overcoming a legendary dwarven warrior without the aid of any spells....