If you want control over polygon reduction then you have to do it by hand, by collapsing edges/merging points. I generally don't use any kind of auto decimation either. Only thing its really good for is preparing models for 3D printing.

You load the displacement map into the displacement map input in the zbrush pallet on the right of the screen. As long as the model has the same UV map that the displacement map was created on then it should work.

Displacement painting is technically sculpting in zbrush. You do it using alpha brushes. I don't use gimp so I don't know how it does things.