It is also possible to simulate its effect via digital manipulation.
One way is something I have called digital GND filters. This was used to create the image on left. Another is the use of multiple images and HDR processing.
In this image I had preferred the outcome from use of digital GND filters on a single image to that from HDR processing.
The basic idea in digital GND filters is to replicate the effect of a GND filter with two Photoshop (R) concepts, namely a multiple blend layer and a gradient mask.
The multiple layer reduces exposure and the gradient mask graduates this reduction along some geometry and direction.
And with digital GND filters, you can, during post processing, play around or experiment with the gradient, the direction, and the geometry, far more conveniently than any "analogue" GND filters can, if at all possible.
Finally you need not just use a multiple blend. Screen and soft light blending modes can also be used singly or in combination, as was the case in the image here.
But there is no free lunch in the real world, and there is a downside to digital GND filters, namely noise. But these can be ameliorated to a large extent by use of appropriate noise reduction software.