It is hard to say what we should exactly do when it comes to tagging, since we're faced with the issue of there being those who perceived Kyururu as a girl - quite a bit of art on here reflects that, while others don't show it as much even if the artist in question may have believed it to be the case (perhaps the only indication, if any, being the usage of "-chan").
I personally agree with a case-by-case approach, where those clearly more feminine would be tagged with girl, while those more ambiguous-looking or male being tagged with boy. Since this isn't much of a spoiler, having spread fast online with many reactions, that seems to be the most sensible route.
I decided to try my hand at it, though I've left out genderswap_(mtf) for the time being. It's a rough start, especially with the usage of otoko no ko. If anyone has any issues with what I've done so far, please say so.