Zoro utilizes CoO haki to an extent, which should allow him an advantage in defending against Gowther's attacks. However, since Gowther's attacks are based off of mind hax and that Zoro's attacks are generally required to be in melee ranged (outside of a few attacks, of course), Gowther takes this due to being hard to kill (even decapitation fails to kill him), and that he just needs to invade Zoro's mind and incapacitate, or even kill him.
Zoro's only possible chance is some-how one-shotting Gowther in a way that will completely leave him unable to attack, but the chance of that happening is almost 0.
Gowther takes this mid difficulty, i'd say.
