[Ygorl]

No, it's a multiplicative effect. Each point of growth/death raises or lowers province population by 0.2% per month. Also, each point raises or lowers province income (which is directly derived from population, for most provinces) 2% per month.
So, the longer you go, the less you decrease in terms of absolute income.

As far as GoNB, I've never cast it, but I always assumed it moved your scales towards growth? In any case, give it a few turns to have an effect.