I want to get a gradient that runs from, say, pure black along the left edge
to pure white along the right edge.

I've tried using the standard background-to-foreground gradient (drawn from
the middle of the left edge horizontally across the image to the middle of
the right edge) with B/W as the colors and the colors at the left edge run
from true black at middle to a dark gray at the top and bottom (with RGB
values of roughly 95). I've also tried drawing the gradient along the top
or bottom edge and diagonally and the results are no better.

How do I get a vertically (or horizontally) aligned gradient that is uniform
from top to bottom at any point on the image?

TIA
Norm