The thing is, just prior to that scene, Aizen was shown to be so powerful his Reiatsu could erase people's existence and he could fuck 3 high tier guys' shit (that were hype up to be big deals) by himself.
But Gin comes out, outsmarts him and does more damage to him than the rest of the cast ever since Aizen's introduction.
If anything, Aizen jobbed to make Gin look cool and strong. And he only lost because of Aizen's newfound immortality, not because of Aizen's strength. And Aizen himself admitted that it was the only time in his life where he was actually scared of dying.