I know nothing about gradients, but wouldn't dithering provide the best results? That way you're truly mixing those two colours and not going through completely different colours based on some arbitrary numerical representation of the colours.

Only if you want a dithered look, which oftentimes you don't.

I guess it depends on the size of your pixels. But as long as your pixels are small enough, I think this is the only way to have a gradient go straight from one colour to an opposite colour without going through either grey or a multitude of other colours.

Pixels are much larger than imperceptible, even on "retina" devices: https://jacksonkearl.github.io/canvas-thing/