You can control the gradient just by making the Near and Far Distances very close together, giving a hard edge to the transition. The problem is that it is that you need that to be "per geometrric shape", your hard edge will still cut partway across some of those parts of the geometry giving a rectangle half filled with grass.
You could do something like create a geometric shape that covers precisely what squares you need to have grass, use the hard edge for distance, and point it to the Distance From that geometric shape - you hide the shape of course, or just set it below the objects in the scene. That way the shape is kind of like a "mask drawn in Photoshop" except you can remodel it in the scene at any time without having to mess around with an external image.