Not sure about filters, but if you just need 1/3 of the screen with a filter, you can mask off the shot, feather the mask, put the masked copy of the shot above the clean version, and apply the filter to the top half. The amount of feathering determines how hard/soft the gradient would be.
Nattress Film Effects has a Day for Night filter with literally a horde of adjustable parameters: www.nattress.com