Buu shitstomps him.
And it seems that you don't get how god ki works. If goku IS NOT using god ki, then when he goes ssj, he's the same as his Buu saga ssj (maybe a little increase because of his tranining). Examples that confirm this:
-Vegeta's ssj being equal to Kyabe's
-Goku's ssj2 being equal to Trunk's ssj2
-Goku's ssj being equal to Gohan's ssj in the latest chapter
If goku does not use god ki, then his base power is the same as it would be without having learned god ki. If Goku uses god ki, then in his base form he's already > Ssj3. Example that confirm this
-Vegeta in his base form >> Ssj3 gotenks. Because here he was using his godly base
-Vegeta in his base form = base kyabe. Because he was using his not godly base here.
-Goku in base is = base frieza (who trained and got stronger). So Frieza in RoF was keeping up with base goku who had god ki, that confirms that Frieza > Buu without using golden form
If they're using god ki, and then they go ssj, then ss blue appears. There's not a golden ssj with godly ki.
So to your thread, Zamazu is only at ssj2 level.. goku was not using godly ki against him, he was using his normal ki. If we talk about immortal Zamasu, who obviously received a power up since he was able to keep up with SS blue after, then he wins.