OKAY, I found it was working with two squares filled with a color. BUT
like in my drawing, now setup linear gradiant filling and this does not
work anymore. Right ?
So, in this case I suppose another solution is used?
I'm not entirely clear on your question. But keep in mind that by default, a gradient has 2 stops. One colour/opacity at the beginning of the gradient and one colour/opacity at the end. (you can add more stops by the way). By default (I think), the end gradient will have zero opacity, meaning it goes from solid to transparent along the gradient, along with the colour change. So click the 'edit gradient' button and check the opacity or alpha channel value for both the start and end stop.
I'm not sure if the above helps or not. Hope it does.