How does Gehenian society provide a critique of our sexual/gender stereotypes?
Wow. That's a big question. Almost the entire novel is devoted to critiquing our stereotypes and assumptions in these areas. That said, the first way it critiques them is by denaturalizing our associations. Giving readers human characters who do not have set genders but you cycle in and out of male, female, and a genderless state explodes the assumption that male and female roles are natural. How could they be in such a society? This sets readers thinking; maybe they are unnaturally fixed in our society? How often do we really act based on sex, as opposed to based on social expectations of gender?
The second major way these are critiqued is through having a viewpoint character fall in love with one of these fluidly gendered humans. This lets us experience his emotional confusion along with him.