I am still so certain that Gortash was a chosen of the Second Sundering, because it would just make so much sense, fit the timeline almost perfectly, and explain so much.
I talked about it a couple of times before, but let me quickly do it again. The Second Sundering happened between 1482 DR and 1487 DR (so between 10 and 5 years prior to BG3). The details do not really matter that much. What matters is, that Ao tried to rewrite the Tablets of Fate, an act that brought with it the possibility of reordering the deities of the Realms, so those deities just created a ton of chosen in an attempt to preserve their powers, due to how the Forgotten Realms divine powers work. This did very much also include the Red Wizards trying to get those chosen into their hands and extract that divine juice from them, but that is less important for Gortash and shit.
What is important is this: it does kinda seem that Gortash was not yet a Chosen of Bane, when he sold Karlach. This is never confirmed in text fully, but I feel it is highly likely for one specific reason: he seemed to still have been in conflict with the Zhentarim when he sold Karlach. And the Zhent are followers of Bane, which is why they are actually begrudgingly following him by the time the game takes place. There is a couple of in game documents from the past talking about how they were in conflict with Gortash over smuggling operation in the past. Until they suddenly were not.
What is noticable about the Second Sundering too is, that a bunch of the Chosen created during it, were not active followers of the gods who chose them. They just... vaguely aligned with their morals.
And to me it makes so much sense. Because more than anything I see Gortash as an opportunistic coward. Like, sure, he made the Banite cult in Baldur's Gate big again. But who says that he did that before he became a chosen. Because the thing is that I do not really see him follow a god without a guarantee to get something out of it for himself. And let's be real here: the gods of the Forgotten Realms do in general not do shit for the majority of their followers. And while I totally see Gortash generally liking the idea of being a tyrant, this man also knows that Bane is a god that most people do not like. So being a Banite without a guaranteed return feels like too much of a political risk. Especially considering the one group of Banites that is very clearly active in Baldur's Gate at the time (the Zhent), is actively hostile to him, and probably will not just cease hostilities based on him going: "Yo, I am a Banite now!"
So to me it would make a whole lot more sense that he was generally morally somewhat aligned with Bane, sure, but not actively worshipping him. Until he became his Chosen by a sheer cosmic coincident, like many of the Second Sundering Chosen.
Unlike many of the Second Sundering Chosen he found out that he was a Chosen. Possibly because of the Dark Urge showing up. And Gortash, being the opportunistic coward he is went: "Oh, I can use that in my advantage."
And let's be honest here: this man would have enjoyed the hell out of getting up to the Zhentarim, all smug, and going: "Oh, by the way, I am the chosen of your god now, so you cannot harm me."
That is to say: I do not think that Gortash was a Banite before he became a chosen of Bane. He became a Banite because he was a chosen of Bane and could use the cult of Bane in his advantage once he was. Not because I think he would have been morally opposed to Bane before - clearly not - but because the risk/reward calculation would have been off too him.
Especially with how much he loves to play himself up as a hero I could actually more see him pretending to be aligned with one of the warrior/heroic gods. Again, not because of morals, but because of how he wants to be perceived.
More than anything Gortash is a politician. It is probably the main thing he learned from Raphael.
