Anyway uh in his current state of affairs, G&W one shots regardless of what happens and Gilgamesh would only be able to hurt G&W using Enuma Elish. But even then, G&W's higher stats and regen (which is probably higher than what Gilgamesh has negated) means that this won't be enough, so only full power Enuma Elish will be enough to kill of G&W. And while it's not absurd to think that he would pull it off, I really doubt he would do so quick enough to prevent his defeat.
So unless I'm missing something, the winner should be G&W.
He doesn't. He just ignores foolish ideas like him dying. It will tell him opponents abilities and he will take that into consideration. It will tell him "dude, he's gonna one-shot you with no effort" and he's gonna be "yeah, nah, I'll be fine". And he's not gonna be fine, thus he loses.
Yeah, I'm pretty sure he just one-shots him. Haven't been paying attention to what base servant durability is, but he shouldn't be able to survive his attacks. His armor can somewhat withstand Merodach, but it still wounds him, so High 7-A attacks are a no go.