I know a lot of boys named Jan. I personally dislike the name and its sound. Too short and hard sounding for my taste. I prefer John or Johan or any other form.
I really don't feel this is more a girl's name than a boy's name. I think only UK, US and Canada people might see it as a girl name, whereas in most other countries where you find the name, it's a variant of John and Ian, so it's clearly a boy name to many!
I know this has been a form of John for a long time, but I think it's too feminine-sounding to use on a boy. It also seems like Jan is too masculine-sounding to use on a girl, so I can't see this becoming popular for either sex.