Gin realizes it. He knows that Aizen's a monster. Aizen's a "god" who can't see past his plans. Aizen doesn't know what it's like to protect someone. He doesn't know what it's like to give up everything for one person. He doesn't know what it's like to love. It actually seems ironic coming from Gin's mouth.
Gin loves too much. It's a weakness and he knows it. Yet, at the same time, he can't forget it. The memory lingers in his mind. But, if Ran-chan is alive and well after this, then he can handle it. That's all that matters.
Even when he sees her crying for him. Even when he hears her calling out his name. Even when his heart crumbles and slowly breaks into pieces.
He's fine.
Love is something that's so strong, but only if no one knows. It's like a double edged sword, a curse. It becomes a weakness, a burden, but only if someone else knows. That's why he can't say it, even if that's the thing he wants to do the most. The moment that it's out, his plans, his revenge, they'll all disappear. Not to mention, if it's out, he's dead.
Ran-chan will be tortured right before his eyes, and if he were to be honest, that's worst than death.
So he doesn't speak. He closes his eyes and smiles.