Read this in a book and was intrigued by it: suppose you have a scene with a bright sky (say sunset) and a dim foreground. Instead of using a GND, use a gobo - use a opaque, black cardboard in the front of your lens covering the bright part. Press the shutter button - it will expose for the dimmer part and with a slow shuttle speed. Make the shot. As the shuttle opens, as it is slow, you have time to withdraw the gobo thus also giving exposure to the top half but with less time. Of course must be done on a tripod and since it is manual, may have to try several shots. The aim is to give the lower half more exposure time and the top less.
Anyone tried this? Results different from a GND filter? Very curious.