We define the gradient direction by a unit vector v. The values at the corners are then obtained by the lengths of the projections of the corner vectors on v. A similar approach can be used to create a gradient background for an Axes.
You can create multiple gradient with different color and apply it seperately since you are using multiple rgba already, you can change it and apply specific gradient to your bar.
I recently added a GradientBarchart to World of Charts dashboard and I would like to share the steps on how to create this chart. I used the Sample Superstore data set. You can find this in Kaggle here. First I placed Region on the Rows shelf. Then I placed Measure Values on the Columns shelf.
Howto Apply a Gradient Palette to a Bar, Column, or Funnel Visualization This article describes how to go from a bar, column, or funnel (pyramid) chart that has a single series of data shown in one color, like this: