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I have not used all of the new modes along with smoothness variations for very long. Yet will share what I know how to use them.

- If you need to have  perfectly linear gradient from a color number perspective, the using the smoothness = 0 and using mode as Classic is needed.  The only need for this specific situation of what I am aware is for mathematical or scientific analysis.  That is a very narrow/specific need.

- In any case where you have a gradient created with several segments (i.e through a preset or custom gradient editor), I recommend that you keep the smoothness at or near 100.  This helps create a smoother transition and avoid the optical illusion of bright spots within the gradient.

- Fro the three modes of Perceptual, Linear, and Classic here is an Adobe helpx link for Adobe's explanation: https://helpx.adobe.com/photoshop/using/gradient-interpolation.html  For me, those explanations are pretty vauge yet that is the best I have seen so far.  After examining what the Perceptual mode does, I personally would use it with caution.  It is the equivalent of moving up the black point (this can possibley cause loss of data in the dark areas by clipping) along with a slight bit of reduced gamma (center slider on the Levels Adjustment Layer).  For the Linear mode that is the equivalent of raising the output level of the blacks a bit and adding a positive gamma (center slider on the Levels adjustment curve).  No image data is clipped and particularly the shadows are lifted.

The above is the technical result of using the various modes to give you a feel for what they do.  Sometimes if just takes using them and see which gives you the desired results.

I hope these details help some

John Wheeler