To me, he's a (stereo)typical hyper teenage fangirl on a sugar high most of the time. Switching his gender would have a minimal impact on both the character's behavior and the story (maybe the media reactions to his relationship with Yuki would be a bit less loud, but even that concerns only one episode).

The thing is, of course, that if Shuuichi was a girl nobody would be interested in the story because then it'd be just another run of the mill romance between a naive but enthusiastic girl and a bastard with issues. (IMO this is why Gravitation is so accessable even for people who normally don't like BL.)