You haven't supported the idea that gender exists any differently from anything else, though!

The only argument you made referenced the lack of a clear dividing line.

But if many things do not have a clear dividing line, then we are not uniquely confused about gender. We're confused about all kinds of stuff. Or, alternatively, maybe you're not properly capturing the nature of the understanding that we do have.

I don't think this is just abstract philosophizing either. I think this directly underlies some of the most pernicious and intractable differences of opinion because people react based on an understanding that doesn't match the usual way we talk about it. Talking then doesn't result in sharing of understanding.

This is why it is worth examining other concepts and categories using the same logic that you apply to gender.