Do we know for sure that the Gods were shut out during the No-God's existence? Certainly the whole all-babies-being-stillborn thing seems to indicate that the souls themselves were not able to go to, or come through, the outside. But perhaps the gods were more involved that we have been thinking?
Well, I think that we have been possibly conflating the No-God's existence and the whole plan itself.
From Kellhus talking to Moe:
“To shut the World against the Outside,” the pale lips said. “To seal it through the extermination of mankind …”
Also, we learned from Bakker that the No-god has an expiration date. So, I think the plan is to have the No-God break the cycle of souls while reducing the population, and once the population is suffiently low, then can the world be sealed, probably through some sort of sorcery that Shae cooked up or something. The No-God itself is simply a tool to get to that point, not a seal itself.