So, rigt now we have a gradient map filter, but it doesn't work on adjustment layers.

Why? Because it isn't thread safe. Random pixels get passed through it twice or more times, resulting in noise.

Now, while I was looking at it, I realised something:

Normalize and Posterize are probably also not thread-safe. However, if you run a pixel twice or more times through these, they'll give the same result as when you only run it through once. So we don't notice they're not thread-safe.

So the real issue here is about making colortransform thread safe.