User blog comment:B214/Kuoh Academy students finding out that Issei stays with the ORC girls and is dating Rias./@comment-24098614-20131215011722/@comment-16281343-20131215081536

With Gasper's disposition, I don't see him as a person that will change his clothes with other people, especially boys.

And also, think how the boys will react when they see Gasper in the changing room together with them.

There is also no sightings of Gasper wearing Kuoh Academy's male uniform, meaning he still wears the girl's. So.. if a person whose features looks like a girl and wears a girl's uniform transfers in a school, the first assumption is she's a girl.

I don't see anyone blabbering about Gasper being a boy and I also don't think they can confirm it since Gasper will most likely shy away from them.