Mihawk is the WSS, with build up for over 2 decades as Zoro's rival. I don't think Gandhi should end up stronger than him, at least current Gandhi.
However, we have to acknowledge that the minimum level of power that is needed to wield a Supreme blade (Only Roger,Whitebeard and Mihawk wield such weapons) from a sword series thats notorious for having cursed blades.
We saw how Enma (which should be inferior to Shodai Kitetsu) gave Zoro trouble, now imagine the strong curse that Shodai would have and the strength needed to control it.