Majin Tenshi said:the armor makes him forget the pain, not the beast. He won't feel anything till the armor comes off.
I would venture that the way the armor makes him forget the pain is by awakening the beast entirely and giving it control. Of course, it provides additional support by reinforcing broken limbs and the like.
What I want to know is, since the armor is piercing Guts with metal spikes in order to keep him upright and going, how the hell is he going to survive its removal? He'll have more holes in him than a sieve.