Briefly going back the tool options at the top of the screen, let's finish setting them up. The second option selects the alignment of the gradient. There are several options, but for our photographic gradient we'll use the standard Linear Gradient. While you're there, set the Blend Mode to Multiply and the Opacity to 75 percent. We don't want the effect to be too dark, so limiting the opacity in this way will allow the background to show through. For the remaining three check boxes, only the one labelled Transparency should be checked.
To apply the gradient effect, position the mouse cursor at the top of the image, click and hold the left button, and drag downward. The direction of the gradient will follow the angle of the line you draw, so if you hold the Shift key while you do this, it will constrain the cursor to draw a perfect vertical line, ensuring an even gradient. It doesn't matter if the line is offset to one side, as long as it is parallel to the sides of the frame.
Since this image is mostly sky, I'll drag the line from the top of the image right to the bottom, although with images that contain more foreground it may only be necessary to drag it as far as the horizon. The gradient effect will only cover the area of the image that is covered by the dragged line.
The result is almost indistinguishable from a photographic filter applied to the camera lens. You can experiment with different colours and options settings, and even with applying more than one gradient to the same image. You can also apply the gradient on a separate layer so you can try changing the blend mode for different effects, but just make sure you've got that safety equipment handy…