What seems to happen is that hard mode gives your chars a 'boost' of -25% HP, but it uses the initial HP for that, so does GC to take another 50%, so one ends up with (100-25-50=25)% of normal mode HP, which is 1/3d of what one normally has on hard, instead of only half as the GC description says. I actually entertained an idea of using it on one of my chars, but 1/3d is just too harsh, I figured I'd just be one-shot most of the time. I'm not sure if it's intentional, but even if it is, I don't think it's right.
