Smooth GIF gradient in Photoshop?

How can I use Photoshop to make gradient dithering go all the way across
an image without bands? JPEG would solve the problem, but I want a
natural gradient in a GIF. The example referenced below was generated in
Photoshop using the Digital Frontiers HVS plug-in, set for high
dithering. Zones that were not Web safe in the original are dithered,
and safe zones are unchanged. The result is vertical banding. I've
experimented with Andromeda Screens but that output is way too rough.
Thanks in advance!

http://home.unicom.net/~kpurtell/hwg/gradient.htm

-- Keith Purtell

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