I'm trying to create a gradient line on a vector layer, but I find it hard to control the gradient.

What I was aiming for, is a line that starts out bright on the left side and goes more and more transparent at the right side. What I got was that PLUS a gradient going top to bottom. (see below screenshot)
How do I control the direction of the gradient?

https://www.dropbox.com/s/82g8q8yusprrzhb/KritaGradient.png?dl=0

Your line is quite thin and that may be causing problems with the gradient. When you select the line and go into the Tool Options docker then choose the gradient option for the line, there's a gradient controller that appears and it has yellow handles with a white dotted line between them. This determines the direction and extent of the gradient. This controller initially goes exactly over the vector line and so you may not have noticed it.

If you grab and drag those yellow handles, you can change the start/end points of the gradient and also drag them above and below the line to give a vertical component to the gradient direction. With a narrow line, the controller is quite sensitive and twitchy and it may have been initialised a bit off from purely horizontal.
If you increase the width of your line, then adjust the right side controller handle, you should be able to get the gradient purely horizontal. Then you can reduce the width of your line to its original size.

For a very thin line, you may be better using a pixel brush so you have no fading at the edges of the line.

Thank you Greybeard !!

I did not manage to get the line perfect, but using a rectangle with fill and no stroke, I could make it considerably larger and get the gradient just right using the technique you mentioned, then make it thin as the line.

Much appreciated.