One needs to show exceptional capacity for knowledge and intelligence within one field to be considered a genius. Read the description for genius.
Individuals with an exceptional capacity for knowledge and intelligence, usually in one area of varying depth.
Gojo meets this requirement, he has an immeasurable understanding in curse energy manipulation compared to people who haven't like several talented sorcerers like Naoya, Megumi, Naobito, Getou, Kashimo, Ryu etc, he has the most complex ct conceptually, is able to do anything he tries to. His time spent in the prison realm also increased his understanding in domain manipulation.
His depth of ce manipulation is nearly the peak of the verse besides Sukuna therefore, he is a genius.
Can check wikipedia for how they describe
genius as well. Its really all about the person's creativity and application within certain fields or field that surpasses expectation of others in it, which Gojo does so.