See:
It seems KICKR Climb declines are cut to 50% in Zwift and this can not be worked around.
I have KICKR Bike and I know it is similar (but a bit different) using the Wahoo SYTSM X training programs. I think you can specify a max downhill gradient, and the default is not very low. In my experience, it’s better this way as a longer downhill can put quite a good bit of pressure on your hands and gets uncomfortable after a bit. IMHO, it’s less immersive and more distracting at steeper downhill gradients compared to uphill, which is more immersive.