There is indeed a Gradient filter and it would be perfect for this task.
Set the blend mode to Overlay. For the color endpoints: black will darken, middle gray causes no change, and white will lighten. In your case, use light black to middle gray. For the bounding box over the video, the vertical height controls the gradient location while the horizontal width controls the rotation (angle) of the gradient. It may be necessary to type “1” in the first Size box to get a gradient level with a flat horizon. The Offset slider changes the softness of the edge. It ranges from smooth gradient to hard-edge line.
For an even darker gradient, use the Burn blending mode and set the gradient colors from almost white (darkens image) to pure white (no change).