This is controversial, i know. But hear me out......
Aizen states that he never thought of anyone else as his vice-captain than Gin.
But Gin makes so many mistakes. And for a genius that finished the academy in a year that's a little surprising.
A few examples.
Aizen makes a plot to let Hitsugaya kill Hinamori.
But Gin attracts the interest of Hitsugaya with that sneaky smile of him.
Also Aizen didnt know that Hitsugaya an Hinamori were related, Gin should have known as he was threatened by Hitsugaya not make Hinamori bleed.
Aizen gave Gin the order to get Hitsugaya and Matsumoto away from room 46, but Kira said, he only had orders to distract Matsumoto.
When Aizen saw Hitsugaya come back, he said that it was not expected.
Gin replied, that apparently Kira didnt work out well, he did not say he order Kira to only get Matsumoto away.
When Ichigo came in while Renji defended rukia against Aizen, Gin felt him coming but failed to warn Aizen. He made some excuse, about not interfering.
Also Aizen wanted to kill Rukia, but Gin asked him if he could kill her.
This little timeframe of him getting in position to strike, gave Byakuya enough time to get her and block the attack with his body.
Because he is a genius, he understands Aizen and probably thinks as fast as Aizen does. Maybe even faster, to counteract in a non revealing way.
In hueco mondo he adjusts the structure of the building, but when asked by Ullquira, he denies... why??????????
Also Kira's explanation of his squad's philosophy makes me think that Gin is the kind of guy, who fights without brute methods.
Disabeling your enemies plans from within is quite sophisticated..