LinearBarVertical fill gradient is same orientation as horizontal LinearBar

using JUCE 3.1.1

LinearBarVertical fill gradient is a vertical gradient (same orientation as the horizontal LinearBar) which gives not the intended visual effect

is there a way to rotate the gradient manually without an explicit user-draw ?

That's just how the standard look + feel chooses to draw it. You'd have to create your own LookAndFeel class and tweak the drawing code to suit your own preference if you want to change it.