Need help with an alpha fade effect

[TragicWarrior]

I found this snippet on which applies a fade effect to the bottom edge of the source image. I would like to know how I could apply this to the left edge (and the others as well).

convert gr.jpg \
      -alpha set -channel A \
      -sparse-color Barycentric '%w,%[fx:h-400] opaque %w,%h transparent' \
      -background '#000000' -flatten \
      gr-gradiented.jpg