For purty, I think you're going to have to paint. What the limits of the
painted area are depends on your esthetics. I have the impression that
there are many paints suitable for high traffic areas. Porch & deck
enamel?? Creative masking of the edges of the paint can make it look like
something that was planned.

Good to know that some paint can withstand traffic. Honestly, I am not
impressed with paint that people put on porch or deck. They eventually
deteriorate after foot traffic especially near the stairway.
Therefore, I try to avoid using paint.

Hopefully, I can find some intermediate agent that I can put between
the cured epoxy and the gelcoat (that I want to put on top of the cured
epoxy), and somehow help them to bond together.

If your existing non-skid is the sand-in-paint flavor, you can probably
match the texture after a few tries. If it's a molded in pattern, then
there is a scheme where you make a "mold" from an undamaged area and press
it down into/on top of the fixing goop before it hardens. Seems like I've
seen a write-up in one of the magazines recently for this technique.

Not sure how the existing non-skid surface was created. I will try a
couple methods to see which one comes close.

Jay Chan