How is this gradient made with AI?

Draw the required path with the pen tool, fill it with dark green, and remove the stroke blending tool. Then, copy one and move the copied object to the lower right corner to reduce the opacity (0-30% as needed).

Using the blending tool, click and drag the first object to the second object, and then release the mouse.

If it is not smooth enough, double-click the blending tool to pop up the dialog box-the distance drop-down arrow selects the specified number of steps-and enter 200 steps to achieve the above effect.