Oh wow I was rereading this chapter too. In Japan there really no difference for he and she, they're the same word over there. But I honestly though Deidara was a girl too.
I don´t think he would have given a damn anyway. Biggest thing he´d do would be to raise an eyebro- oh wait, he doesn´t have eyebrows..... Anyway, would be a little bit funny if he´d mistaken him for a her, but who knows.
Gaara probably didn't think he was a girl, but the translators. I don't think Deidara's gender was specified yet so they may have believed he was female