I'd have to say no. Anything good in Griffith was burned away when he became a demon. That aside, the human Griffith decided freely to give himself over to damnation at the cost of his friends' lives. Realistically, there would be no hope of coming back from that, and from a storytelling aspect...
Re: Why does Zodd help Gatts?
I have a theory on this. I don't know if anyone's suggested something similar before, but here goes. For one thing, we know Guts' mother was dead when she gave birth to him, but we know nothing of his father. I think that Zodd is Guts' father. There are things that...