Gender Apathetic or Apagender is when a person really doesn't identify nor care about any particular gender. They are fine passing off as whatever and do not have an opinion towards their own gender. This is different than agender in that they DO recognize that they have a gender, they just don't particularly identify with it nor care for it. Its common but not a lot of people know about it. It could be said as multiple terms like Trans Apathetic, Nonbinary Apathetic, Cis Apathetic (etc.). The terms that was listed are what the person would lean more towards. But a person doesn't need to be one of the terms was listed. They might say "Eh...I don't really think about it but I guess I'm..." to a person who asks them their gender. Most people don't know that gender apathetic people exist because they aren't really out there with their gender.
Gender Apathetic can be confused with Cassgender, which is when someone feels their gender is unimportant. The difference between Apagender and Cassgender is that Apagender people don't care how people gender them, whereas Cassgender means that the people don't think their gender is important