It may be the way you have your icc options set. Controlling GCR is important. If the banding is caused by too much ink you can use more K and less CMY to create your darker areas. Lower the point at which K comes in and adjust the curve so that you dial out CMY as appropriate.
You could also try UCR - dark areas this time START with the K and you add CMY to it instead of the other way round. This will give you less ink in your blacks and more neutrality (but it may look more grainy)
If all this is too much hassle, simply use the CMYK Contone path - you probably won’t get banding as the printer controls all this. (avoid RGB Contone - too limited!) The drawback is you are not in charge of ink restrictions so your densities and or gamut may not be 100% optimised. The Contone (as opposed to normal Halftone) path is available in the very latest drivers at www.onyxgfx.net, if you are on an earlier Z6100 driver.
When Contone profiling you skip straight to the calibration, missing the ink restriction step. Once built, you can “next” to the ink limits as usual but it is not necessary - just jump over this part by clicking on the icc stage in the wizard list on the left hand side to go straight there. Build your icc as normal using the printer and you should have a good profile with minimal banding.
Remember to use the CMYK Contone ink config and keep the pass rate as high as possible, according to the published matrix. Also base the profile on one of the HP Art Paper media types.
Hope this helps!