The gradient map filter is definitely useful, but what would go significantly further in my opinion is an adjustment layer.
When the gradient map is above your heightmap as an adjustment layer, you can paint on the heightmap and see it update in real time with the gradient mapping applied.
This is hugely beneficial in game art, and texturing, where we are constantly using gradient maps and heightmaps to reduce our memory footprint.
Painting on the heightmap "blind" though, is very difficult, not being able to see what the result is stroke by stroke. This becomes an even bigger problem if you are using gradient maps someone else has already created, and need to author your heightmap to work with specific predefined gradients.
Also, the filter is destructive. Once you've committed it, it's committed forever. Adjustment layers offer you the flexibility to make corrections at any time. This is hugely useful for any industry/application.
I apologize if it sounds like i'm bashing the functionality. I'm certain you've worked hard to implement it. Though Paintstorm and it's brush engine are such a joy to work with, not being able to utilize that power for the majority of our textures is a shame.
If I had one wish for this program it would be gradient mapping adjustment layers (and being able to save the gradient out to share with P.S.).
I've attached an example section of a tiling heightmap of mine, with a few variations of gradient mapping, to illustrate what the point. I painted this in photoshop a year or two ago, and I think i turned ok, for being painted in PS. I can only image the improvement we'd see if we could do the same in Paintstorm
Thanks for your time,