Cassgender is a gender identity were one feels that their gender is unimportant. Or when one is indifferent to the idea of gender. Cassgender is different from a lack of gender, as a cassgender individual might have a gender, but they don't feel it's an important part of their identity.
A related term is cassflux, where the level of indifference fluctuates. Sometimes one might feel completely indifferent towards gender but other time it might feel more important.