They keep calling Crona an it, so I'm not so sure myself. Hopefully, it's a girl. But in the anime and manga nobody looks like they know it's gender, but from the looks of it and how close it is to Maka, once it hits the US, it's going to be a he. It would make a good argument though. I have some strong statements it's a girl. I mean in one of the episodes
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After the fight they had with Crona. When Ragnarok tries to look under it's dress or whatever, and it kept calling him a pervert, I guess that kind of gave me a strong feeling it was a girl.
Plus, it has pretty nice hips xD So my answer falls as a girl.