what is the gestation period of a dark one?
Probably a combination of as long as the mother wants it to be, measured against how long the fetus takes to develop some sense of sentience. Dark ones are shape shifters so if a 12 week dark one fetus gained self awareness then I imagine there'd be pretty much nothing stopping them from just developing themself as much as they want.
Will the new one be a primordial?
Possibly? The only new dark ones in existence are Noel and Amy and they were both instantly primordials. Were they primordials because they were created by a primordial or were then because they were created by Nael? Amy might be a primordial but she's way weaker than Nael so who knows if her offspring would exceed the authority threshold of a primordial. I suspect that her offspring would be an extremely powerful dark one, likely high S class potential but I'm skeptical that without some serious intervention or feeding that she'd exceed a primordial's baseline.
Will the MC defect to the Brotherhood, and can he take Emma and Clara with him?
Probably, depending on your stats. I've got 81 brotherhood points on my main save, it'd be weird if MC didn't join them, or possibly even promote to leadership of it. Then again who knows if there'll even be a brotherhood after what that shitbag Claire did.
Is there a way to get Gab back in the game?
Last time Gab died Nael destroyed the entire fucking universe. I think he'll die at some point but not until Nael mentally and emotionally develops a bit more. He'll probably get rescued at least this once.
What I find really interesting is what will happen to Two. MC and Blake aren't going to fight him on a busy passenger airplane when all 4 of them need to get home first, but they're gonna be locked in tight quarters on a trans Atlantic flight. That's a lot of time to bury the hatchet and Two is by far the most pragmatic of the 4 horsemen that we'd seen.