Genderfuck can be either a verb or a noun.

To genderfuck means to play with gender expressions with the express intent to confuse other people. Many people can't handle someone who doesn't conform to one gender but uses aspects of both male and female gender to express hirself.
Thus, a genderfuck would be someone who engages in genderfucking.

A good example of a genderfuck would be Brian Molko. He is male, and likes to wear female clothing and makeup, but he doesn't really try to look like a woman . The result is very disturbing to some people, and he seems to enjoy that.