So, Gabriel is/was Josephine's partner (as shown in the video footage in this episode, and flashbacks in past episodes) and the faction of people who oppose the Primes are called the "Children of Gabriel".. So based on clues dropped in this episode, and some past information from the season I sort of have two theories...
Firstly, I think that the "old man" being referenced/leading the Children of Gabriel is actually Gabriel, in his last host body before he split off from the rest of the Primes. SOMETHING must have been the catalyst for that decision, though - we see him looking a little conflicted about his success in bringing Josephine back in the footage from this episode, and I think that's meant to be our indicator that he wasn't 100% on board with the whole host body/immortality thing.
So theory one is that Xavier is the child of Josephine (in her last host body) and Gabriel - he's hinted at having some sort of authoritative presence in the children of Gabriel but doesn't seem to be the outright leader, and if he's the child of Gabriel himself that would seem to make sense in terms of his positioning within the group. I think the turning point for Gabriel could have been having a child, probably with nightblood, and fearing that his child would then in turn become a host when his child reached the right age of maturation (because Josephine seems just crazy enough to wipe out her own kid in order to further the rest of her family). Knowing that you're raising your kid 'for the slaughter', while also having an INTIMATE knowledge of what that would entail would definitely be more than enough motive to split off from the Primes...
Theory two hinges around the "Isaac" that was mentioned in this week's episode. As far as I'm aware, we have no idea who Isaac is - just that he was important to Kaylee. So, similarily, my theory here is that Isaac was the nightblooded child of Kaylee/her husband, concieved using their host's bodies. I think Josephine, against Kaylee's wishes, used Isaac as a host for a different prime (maybe Gabriel?) and that was both what motived Kaylee to then kill Josephine (for having killed her son) and also acted as the last straw for Gabriel, causing him to split off. Would also be good motive for Kaylee & family to run away, since she would know that when Josephine is "reborn" again she'd be able to tell the rest of the Primes than Kaylee was the one who caused her fall.
Purely speculation of course - but fun to think about what the different clues might mean and try to guess at how everything could come together!