Axial ("linear") or radial geometry.
[x0,y0,x1,y1] for "linear", [x0,y0,r0,x1,y1,r1] for "radial".
At least two colour stops.
The rectangle { x, y, w, h } filled with the gradient.
OptionalextendExtend before the first / after the last stop (/Extend; default [true, true]).
OptionalopacityFill opacity 0…1 (default 1).
A gradient fill for GigaPdfDoc.addGradient.