Gradient Drawable works the same as our xmlshape, supporting gradient, rounded corners, etc.

Look at the construction method first.

            mDrawable = new GradientDrawable(GradientDrawable.Orientation.TL_BR,
                                             new int[] { 0xFFFF0000, 0xFF00FF00, 0xFF0000FF });

orientation specifies the direction of the gradient (the following eight kinds). The color of the gradient is specified by the colors array. Each value in the array is a color.

 /**
     * Controlling the direction of the gradient relative to the renderable boundary
     */
    public enum Orientation {
        /** Draw gradients from top to bottom*/
        TOP_BOTTOM,
        /** draw the gradient from the top-right to the bottom-left */
        TR_BL,
        /** Draw gradients from right to left */
        RIGHT_LEFT,
        /** Draw a gradient from the lower right corner to the upper left corner */
        BR_TL,
        /** Draw gradients from bottom to top */
        BOTTOM_TOP,
        /** Draw a gradient from the lower left corner to the upper right corner */
        BL_TR,
        /** Draw gradients from left to right */
        LEFT_RIGHT,
        /** Draw a gradient from the upper left corner to the lower right corner */
        TL_BR,
    }

Three gradient modes, Liner,Radial and Sweep, are used to show the different uses of Gradient Drawable by specifying the radius of four corners and rounds of a rectangle.

@Override protected void onDraw(Canvas canvas) {

            mDrawable.setBounds(mRect);

            float r = 16;

            canvas.save();
            canvas.translate(10, 10);
            mDrawable.setGradientType(GradientDrawable.LINEAR_GRADIENT);
            setCornerRadii(mDrawable, r, r, 0, 0);
            mDrawable.draw(canvas);
            canvas.restore();

            canvas.save();
            canvas.translate(10 + mRect.width() + 10, 10);
            mDrawable.setGradientType(GradientDrawable.RADIAL_GRADIENT);
            setCornerRadii(mDrawable, 0, 0, r, r);
            mDrawable.draw(canvas);
            canvas.restore();

            canvas.translate(0, mRect.height() + 10);

            canvas.save();
            canvas.translate(10, 10);
            mDrawable.setGradientType(GradientDrawable.SWEEP_GRADIENT);
            setCornerRadii(mDrawable, 0, r, r, 0);
            mDrawable.draw(canvas);
            canvas.restore();

            canvas.save();
            canvas.translate(10 + mRect.width() + 10, 10);
            mDrawable.setGradientType(GradientDrawable.LINEAR_GRADIENT);
            setCornerRadii(mDrawable, r, 0, 0, r);
            mDrawable.draw(canvas);
            canvas.restore();

            canvas.translate(0, mRect.height() + 10);

            canvas.save();
            canvas.translate(10, 10);
            mDrawable.setGradientType(GradientDrawable.RADIAL_GRADIENT);
            setCornerRadii(mDrawable, r, 0, r, 0);
            mDrawable.draw(canvas);
            canvas.restore();

            canvas.save();
            canvas.translate(10 + mRect.width() + 10, 10);
            mDrawable.setGradientType(GradientDrawable.SWEEP_GRADIENT);
            setCornerRadii(mDrawable, 0, r, 0, r);
            mDrawable.draw(canvas);
            canvas.restore();
        }

It is also allowed to set four corners of a rectangle as rounded corners. The radius of each corner can be set separately:
public void setCornerRadii(float[] radii)
Radii arrays specify the radii of four rounded corners, each of which can specify [X_Radius,Y_Radius]. The order of the four rounded corners is upper left, upper right, lower right and lower left. If X_Radius,Y_Radius is 0 or right angle.

        static void setCornerRadii(GradientDrawable drawable, float r0,
                                   float r1, float r2, float r3) {
            drawable.setCornerRadii(new float[] { r0, r0, r1, r1,
                                                  r2, r2, r3, r3 });
        }