I went back and checked, too. You’re right. It is Gin who opened the gate. So, if this is anyone’s “home port,” it’s probably Gin’s.
Aizen is all over Gin earlier, too. Pretty much any time Gin has his hand on Shinso, Aizen is “What are you doing, Gin?” so Aizen must mentally preparing for the betrayal he claims he knew was coming (and I actually believe he probably did, especially given how often he’s all, “What are you up to??” with Gin in these final chapters.)
I wish we knew where Aizen was from. We know what he wants: to be the Soul King and for “a better world.” But we don’t know why. We don’t know when or how Aizen saw the Soul King and determined him to be the “Empty Throne.”
The only other clue we have is that Aizen doesn’t know that people in Rukongai celebrate birthdays. He’s surprised that Hitsugaya celebrates one and asks him about it.
Honestly, Aizen seems to know shit about things in the Soul Society. He doesn’t know about Zero Squad (he’s really shocked to discover that captains can be promoted to the Royal Realm. He says he assumed they became members of Central 46.) So, if he was born inside the walls of the Seireitei, how did he miss this stuff? He’s also all over the archives of Central as soon as he gets inside. He’s searching for Soul King stuff, but again… it’s amazing all the ways to get to the Soul Realm we’re discovering now that have nothing to do with the oken.
I really hope that if Bleach is ending soon and there is a spin-off, we get more answers to this stuff.
Otherwise I’m going to have to make it up.