It doesn't make sense either way, Imu had a taste of Rogerbeard + Xebec + Garp (+ rocks duo) combo attack and he still surprised by Gaban strenght.
Are we mean to believe Old Gaban is a whole tier above all of these guys even if they weren't in their exact prime? I mean, I don't mind as it is more Kizaru upscale but it still feels off