It was always there in the Manga, Kubo didn't change a single thing between the Anime and the Manga or put it in a Databook(The only thing he did was add context that what Aizen took from Rangiku was a good chunk of her Soul and that reason she didn't die on the spot was because she is some part have something related to the Soul King within her). That's why Gin wanted the Hogyoku from Aizen because it was for the missing chunk of her soul Aizen took from her and to kill him for doing that to her. Otherwise yeah, Gin was never straight up blatant about it but even against Ichigo, there is heavy vibes he kept Ichigo alive precisely because he was the only other person past him that could kill Aizen(Gin could have killed Ichigo at MANY points but refused to).