So it took me a while to come to this conclusion, but after going over a few chapters again I now fully understand why Gin attempted to kill Aizen at the time he did and his reasons behind it. First off, it is important to note that what was “stolen” from Rangiku is up to interpretation, it is only to be used as a symbol, in this case to show just how much Gin loved Rangiku and just how far he would go in order to get his revenge. Gin had two main goals: to kill Aizen and to steal the Hogyoku. The first goal is obvious, Aizen was the one who stole “something” from Rangiku. This ties into his second goal, because what Aizen stole from Rangiku eventually became part of the Hogyoku. His wish was to get back what Aizen had stolen from her, and in order to get it back, he had to kill Aizen at the right opportunity, and what’s not a better opportunity than Aizen being fused to the Hogyoku? So after many many years, Gin had finally found the best opportunity, and by knowing Kyoka Suigetsu’s weakness and having an ace up his sleeve (Kamishini no Yari’s poison attribute), he finally accomplished his goals after over a century of torment, all for his love of Rangiku.
He now held the Hogyoku, containing “something” from Rangiku, and killed the one responsible. But in the end, Aizen is not really killed as the Hogyoku brings him back to life and grants him even greater power, with the fear of being killed by Gin as the catalyst. Gin ultimately fails, as Aizen strikes him down and retrieves his Hogyoku, which therefore means he has failed to retrieve what Aizen “stole.” However, moments before his death Ichigo appears before Aizen, and Gin sees that hope is not lost as he believes Ichigo now has what it takes to win.
Now as for what was “stolen,” we pretty much have another briefcase from Pulp Fiction. We don’t know what it is in the end, but we do know it’s important. It is known that Aizen was taking souls from Shinigami as well as citizen’s of the Rukongai with potential to become Shinigami, but I don’t believe that’s what was stolen from Rangiku because well, she’s still alive. What I personally think Tite Kubo had in mind was love. Love. He claimed for it to be a “real find,” and if you think about it, it makes sense that something like this would drive Gin to such extreme lengths. Gin wanted to take back the love that was stolen from him. If they had a history together, why did they never seem to have a relationship after the incident? Rangiku still cares for Gin, but could never love him because that was in fact what she lacked. That is why she cried, and why Gin wanted to stop her crying. Love is worth fighting for.
Sorry haters, but Tite Kubo is a genius writer, as well as a genius artist. I never fully understood what he was trying to portray at first, but after breaking everything down it all makes sense to me. A truly beautiful story.