Sorry to ask this, it may sound stupid. But I see no difference in gender bender and crossdressing, aren't they the same thing?
Yes, I think there is a difference between these two conceptions, though only subtle in genres of mangas. I think someone who cross-dresses in manga does this because it's their hobby. They wear clothes of the opposite sex.
Gender bender is when someone physically changes their whole appearance, including hair-styles, clothes, voice and personality, etc to become/act/pretend like the opposite sex. Take Hana Kimi or Power!! for example and you can see the difference.