The bassist and vocalist for Atheist isn't in that band, the bassist is what REALLY brought most of the jazz sound to Atheist (particularly in elements), and Kelly brought a lot of the heavy-ness as well simply in terms of vocals.
I think Gnostic is a good band though... I wouldn't say it has 3 core members, just two, the drummer and a guitar player. They weren't the primary writers by any means. I don't think they are at all resembling what we will be hearing from Atheist.