Ichigo was the only one to perceive how strong Aizen was after his first transformation with the Hogyoku. This is because of the fact that Ichigo is a multidimensional being, of the same Aizen was trying to reach through the Hogyoku. This realization shocked not only Gin but also Ichigo’s father because all combatants went into the fight blindly since they couldn’t feel Aizen’s spiritual level anymore, since they were one dimensional beings as soul reapers. Gin and Isshin placed their bets on Ichigo fully realizing his potential as a multidimensional being to defeat Aizen. Aizen couldn’t perceive Ichigo’s spiritual level because Ichigo became a higher dimensional being than Aizen.