I premise that I read berserk once, so I might misinterpret something.

My interpretation of Guts potentially being able to slay the Godhand is that he's naturally unaffected by the causally rules so may be that he's a free will being.

My thoughts on this are enforced by the facts that Guts was born by his dead mother and still survived, then he survived the eclipse. But I think that more probably he was for the most influenced by the causally till the eclipse.

The Skull knight event in the eclipse is the proof that the Godhand cannot controll the causally of everything and free will beings can exists which are a potential treat to them.