Blurring

The -blur command provides one of the more straightforward ramp techniques; the result may not be strictly linear, but the setup is straightforward and intuitive. Essentially, one splits along the x or y axis and blurs to a degree that suits.

#Split, fill, append, blur.  gmic 256,256,1,1 -split x \    -fill_color[-1] 255 \    -append x \    -blur 127 \    -normalize 0,255