+1 for the "most crocodilian of the Sons" being Ajokli, especially as it matches Cnaiur's swazond.
Even though the Gods cannot see the No-God, they can still identify those mortals who were most prominent in the world right before it all went dark from their PoV. So if, from Gilgaol's PoV, the final Apocalypse looks like Kellhus ruling the World, marching a huge army into the North and then *bang!* the Outside is shut off, he's going to make the assumption that Kellhus is to blame and try to avert it through Celmomas' dying vision. The similarities of the two situations (massive army called the Great Ordeal, led by an Anasurimbor, getting slaughtered in the North of Earwa) presumably made it easier for Gilgaol to get the message through at that precise moment.
The vision is not Gilgaol saying, "Here's the schmuck who will unleash/become the No-God." He's saying, "No-God? WTF is that? Everything is fine in your time period, so quit yer whining. Now listen, here's the guy you need to worry about. The real Apocalypse is something to do with him."