The gender is a more or less unnecessary piece of information around here. Even if you wanted to provide your pronouns, no one would be able to see this preference when replying in a thread. It's not in the quick info dialog when hovering over the poster's name, and by default profiles are inaccessable to all, so no one would be able to see that info in the first place. You could use the Location field to enter those, it's visible directly under your name, but that'd be displayed with a Location label, see the example on the left under my name (and compare it with how I formatted it in the profile).
You must also be advised that this version on IP.Board comes from a different internet era altogether (~10 years ago, I believe), one that didn't have the custom of providing pronouns, or having a variety of genders to identify as.
So, you may treat Not telling simply as virtually equivalent to non-binary.