If you use a number of track splines parallel to each other, you can easily get good results. Scenery splines don't have a Gradient tool, so actual rail track splines are best for this type of work.
Space the tracks equally by a distance that is about the same as the terrain grid resolution (usually 10m). Each track should have only a start point and an end point, and the whole set should form a rectangle (the track segments all being the same length).
In the "Advanced" section of the Track tab, there is a row of controls devoted to Gradient. Type in a value of 2.00 (which means 2.00%) and click the "Apply Gradient" button, then click on each track.
Use the "Smooth spline height" tool on each track to form the ground ramp under the tracks.
It should produce a smooth/flat rectangle, sloped at 2.0% gradient, even if the tracks are not aligned with the ground grid (as in my quick example below). Check it in Wireframe mode (F9) and confirm the flatness. If there are grooves between tracks, then the spacing was too large, so move the tracks closer together and Smooth again.
Asphalt parking lot formed by tracks with 2% gradient. I've added some parking space splines for fun. The fences are only there to highlight the cross-sectional profile of the ramp at 2 places.
Wireframe view (F9). This is a 10m terrain grid. The distance between every second grid line is 10m.
With some practice and adjustments to the procedure, I think you could create patches of flat, sloped ground that have shapes more complex than rectangles, but I haven't tried that.