It shouldn't be that hard to find because the literal definition is genderswapping. Whether it's the main character or a supporting character changing genders, a manga is usually listed under the genre "Gender bender" if gender swapping is heavily prevalent in the plot. And I mean by genderswapping is when a character changes genders. If you're still confused, I'd say read one. It's not that bad of a genre, just like any other genre.
i'd say a manga where one person dresses and behaves in a characteristics of the opposite sex
It's when someone looks and sometimes acts like a person from the opposite gender.
Example: in the second picture are two female characters, but on of them looks like a man.
http://www.mangago.me/read-manga/ze/an/ze-chapter-1.html/14/
Either one of the main characters cross-dresses or they actually get changed into the other gender by some means.
It can also cover body swapping, like Freeky Friday but between the opposite sex or transformation into opposite sex by magic / curse etc...
Example
http://www.mangago.me/read-manga/fukigen_cinderella/
I've already read several ones, like Kanojo ni Naru hi !
Haha, I know this one xD It makes it easy to understand ^^ Thanks !
I see, thank you for your explanations ^^ I understand better now !
What's Gender Bender? oO I always search for its definition but never find a satisfying one !