Keep focusing on the Appearance panel and add a second fill using the Add New Fill button (pointed by the blue circle in the following image). Select this new fill and add the complex, linear gradient shown in the following image. Keep in mind that the white numbers from the Gradient image stand for Location percentage. This simply means that you have to select your gradient slider, focus on the Location box (from the Gradient panel) and insert that number. You can easily add a new gradient slider for your gradient by clicking on the gradient bar.