Applications do not agree how to draw a gradient and current descriptions are hard to understand. I have attached a document with "square" and "rectangular" gradient and a document that contains screenshots of the applications to which I have access. Screenshots of other applications are welcomed.
In addition, it is not clear how these gradients are drawn, if the attribute draw:concentric-gradient-fill-allowed (19.124) has value true.
I suggest to add a sample of the expected gradient to each of the kinds.