The real reason is still related to Tsuna Yayoi.
After his rebirth, Gosuke Shigure subconsciously severed ties with his previous life. In that past life, he had been Tsunayashiro Shusui, a noble of the Central 46. But in this life, he is merely Gosuke Shigure, an ordinary Shinigami.
It was precisely due to this disassociation that, even after learning that Shutara Senjumaru still lingered in the Soul Society, silently holding on, he never went to recognize her. Despite his complicated emotions, he kept his distance.
After all, the original Tsunayashiro Shusui had long died.
One might call it cowardice, but in truth, Gosuke had been avoiding it. Avoiding the weight of the past and the complexity of his identity. It was only because he had gotten drunk that night that he summoned the courage to speak with Senjumaru and vent what had long burdened his heart.
After that, once sober, he found himself once again caught in turmoil. Still, that brief exchange made one thing clear: the past can't be severed completely.
Even though he was now Gosuke Shigure, all the memories of his past life remained. The lifetimes he'd lived overlapped and swirled in his mind. Sometimes, even he couldn't tell whether he was just a normal young man from Earth, a noble from the Tsunayashiro clan, or simply the current Shinigami known as Gosuke Shigure.
It was only after Senjumaru Shutara ascended to Division Zero that he began to try and understand what all of it truly meant.
Escaping the past would not work.
Wearing the death costume that Shutara had once left behind, he could feel its difference, its weight. The more he wore it, the clearer he became on the matter.
If Shutara could silently wait for him for thousands of years, even after his death…
Then why was he so eager to cut ties with his previous life?
⸻
Amid the growing conflict between noble houses, Gosuke Shigure indeed began to feel a need to settle things with the Tsunayashiro family.
Of course, to avoid unnecessary trouble, even if one wanted to purge the past, it had to be done at the right time.
One such opportunity would be when Aizen eventually slaughters Central 46.
Though that day remains distant, it doesn't matter. Gosuke had already lived through so many years—time no longer held much meaning for him. He could wait.
In the meantime, after accidentally learning about Tsunayashiro Tokinada, he didn't mind stirring trouble for the Tsunayashiro clan.
That's why he manipulated the Kuchiki family into embarrassing the Tsunayashiros.
At the time, Tokinada, a mere lesser-seated member of the Tsunayashiro branch house, was insignificant. If nothing else, he should have become a scapegoat, quietly abandoned by the clan.
But fate is never so simple.
Tokinada didn't become a sacrifice.
Instead, he turned the tables—massacring his entire family.
For a time, Gosuke Shigure couldn't find the right words to evaluate Tokinada.
In truth, Tokinada had done what Senjumaru and countless others had always wanted to do.
And it was because Tokinada had taken such extreme action that Gosuke knew: he had to step in personally.
⸻
There was no longer any room for mercy. Tsunayashiro Tokinada had gone too far. The Soul Society could never tolerate a man like that surviving.
Of course, Gosuke could've chosen to step aside and let someone else bring Tokinada to justice. But in the end, he made a decision:
He would personally deliver judgment.
To face Tokinada and sever his final ties with the Tsunayashiro bloodline.
Even though he had already come to accept, after Senjumaru, that not all things can be cut off cleanly — this would be his own symbolic act of closure.
⸻
To reach Hueco Mundo, they first had to pass through the Dangai — the Boundary.
This boundary world exists outside the flow of time and space. Time moves differently here. A moment in the real world, or Soul Society, can stretch into days within the Dangai.
In the original timeline, Ichigo Kurosaki once trained for what seemed like a long time within the Dangai before unleashing his Final Getsuga Tenshō. Similarly, Aizen also spent several months inside the Dangai while moving between worlds, which was why his hair had grown out by the time he faced the Gotei 13.
From this perspective, the Dangai seems like an ideal place for training.
But it's not.
⸻
Inside the Dangai dwell two powerful forces: Kōryū and Kōtotsu — time-space constructs that cleanse intruders. These 'cleaners' eliminate anything lingering within the Dangai for too long. Unless one is exceptionally strong, encountering them means certain death.
Thus, it's not a safe place for cultivation.
However, that doesn't mean Soul Society avoids it entirely. On the contrary, they've conducted many experiments inside.
For instance, Kenpachi, during the early days, once tested the might of his Bankai within the Dangai because it was too dangerous to unleash in Seireitei. As a result, a formal ban was instituted, preventing Bankai use in sensitive areas.
Whenever a Shinigami is dispatched to the human world, Soul Society carefully scans the Dangai in advance. Only after confirming it's safe does the Hell Butterfly guide the Shinigami.
But when traveling to Hueco Mundo, there is no Hell Butterfly. The route is made via a Senkaimon crafted by the 12th Division.
⸻
Gosuke Shigure brought Zaraki, while Shihouin Kongchan brought Yoruichi.
Together, the four of them stepped into the Dangai, en route to Hueco Mundo.
Though only two captains were officially assigned — Gosuke Shigure and Shihouin Kongchan — Captain Yamamoto placed no restrictions on whom they could bring.
Yet, in the end, they each chose only one person.
It wasn't surprising for Shihouin to bring Yoruichi. She had matured, and it was clear she would soon inherit her family's legacy. This mission would serve as valuable experience.
But Gosuke Shigure bringing Zaraki Kenpachi? That was more spontaneous.
Initially, he hadn't planned to bring anyone.
But then he thought: Hueco Mundo is full of combat opportunities, and Zaraki, a battle-maniac, would thrive there.
Perhaps a place like that could finally push him toward greater strength.