Most of what I wanted to say has been said already. I'd like to add that her kid still seems pretty young and supposing Gon does end up with Kaori and has a good relationship with her and her son, he'll be more of a dad to the kid than the bio dad. Out of the millions and millions of couplings out there throughout modern human history, surely you can point to at least one example where this has happened and it also working out.
I keep flicking back and forth between the pages before and after the "twist". Might seem obvious, seeing her carrying the kid, but it's amazing how "mom-ly" she seemed right after the reveal, compared to just a few panels before when she still seemed like, to my mind, that fun, carefree girl. I kind of see Gon's perspective, just a little bit. Compliments to Wakaki-senpai for their art as always. Her smile on page 17 expresses so much more than just her emotion in that moment, but also says a lot about her experiences in the past, in this same situation.
This manga just keeps getting better and better. I don't watch/read a lot of romance, maybe because they're precisely the opposite of what this manga is so far. About people disappointed in not seeing the "main" couple more often, while yes, a story needs its protagonists, in real life there's no "main" or "side" characters and plots. For a more grounded story, for realistic worldbuilding, I think it's good that the focus is not always on the protagonists. It's brilliant that the "side" characters don't see themselves as such. Everyone, you, me, we all are the main characters in our own worlds. Gon has initiatives and desires. He's a main character in his own right. And things are happening in this story that our "main" character (Takuya? I keep forgetting his name) don't control. That's great. That's life.