Q1) yes gojo have six eyes I don't even know why you ask this if you read the series Q2)six eyes allow the user have insane perception upto atomic structure and help them in see and have better understanding of flow energy of cursed energy. Six alone does not make you strong but combination of six eyes+limitless what make gojo strong Q3) gojo does not have full understanding of sukuna cursed technique while sukuna have full understanding of gojo techniques.so when gojo talks to geto about sukuna holding back he talks about two things a) sukuna flames which he didn't use during domain expansion b) sukuna Heien era form Because of this gojo thought sukuna was holding back which was wrong a) sukuna could not use flames because of gojo since he have change the condition of domain continuously b) his Heien era form was backup since he knew everyone gonna jump him after gojo death if he used during Gojo fight it will make the fight easier but his other fight will be lot harder than now Q4) so the reason he said this because he didn't knew how sukuna cursed technique work because low information If you use Heien era sukuna who don't have any knowledge about gojo cursed technique same with gojo then gojo will win against sukuna