when selecting a gradient with transparency, e.g. neutral density in the gradient map tool (in an adjustment layer), transparency is ignored, i.e. neutral density is working like plain black - which I assume is not intended. And, I'm almost certain, the gradient map properly applied transparency earlier but I might have changed involuntarily some setting. So far, I could not find a good answer. Any ideas?
No, Gradient map has never applied transparency.
And it sounds like you should look up what the gradient map adjustment actually does.
@R_Kelly, Chris: many thanks for your answers - in fact, I was suspecting, that this is rather a "feature" of gradient map than a "bug".