Often discussed as "The book with the genderless aliens" it is obviously a lot more than that. It does of course touch on the gender thing, as the people of Gethen (Eternally winter planet) have no gender and only enter Kemmer occasionally when they become sexually active and can be temporarily male or female.
It is more a political and anthropological reflection about duality and unity. A theme that maybe starts on the sexes thing but goes on to war and peace, nationalism and the Ekumen (A intergalactic union of planets)
Essential reading though, and awesome world building!