Well, that is a different story. I do not think that was the question the OP asked. We usualy talk about normal ND filters here not gradation filters. I used graduated filters on my DSLR film camera too but those were the large rectangular...
For that to work you'd need to use very long exposure of several seconds or even minutes with camera on tripod. While certainly interesting method, using a drone for this would pose a several challenges. The major one is the motion blur. Drones...
I think ND filters are useless if you plan to shoot stills only. Camera sensors are biased to assume an overall scene reflectance of 18%. If you put on an ND filter. the camera will simply interpret the scene as too dark and will adjust shutter...