Gradients are design tools. The gradient interpolation function can be designed around some physical mixing hypothesis but there is no first principles rule as such.

Focusing on the luminosity is only half of the battle.

For nice looking gradients that are pleasing to design with, generally for aesthetic reasons you also want the perceived color saturation to be interpolated in a predictable way.

For reference, here is a nice example of an approach to find a nice looking mixing function: https://bottosson.github.io/posts/oklab/