Here's a simple pipe (cf the screenshot). I drew a very long, very slim rectangle with a linear gradient in it. This will only work when the gradient is EXACTLY PERPENDICULAR to the long side of the pipe or else the gradient bar will “wander off” towards the top or bottom.

For all I tried, I was NOT able to get my gradient bar exactly perpendicular to the long side of the box by dragging the gradient with the mouse: The length of the gradient is tiny compared to the long side of the pipe, and if I zoom in then I can't see the pipe ends any longer to check where the gradient bar is lined up.

So in the picture attached, the bar indeed wandered off towards the top, see the detail at the right, an effect that I want to avoid..

Hold ctrl-key down when dragging the gradient. It goes in 10° constraint increments afaik.

Thanks, that did what I was looking for. 👍