Hi saDdyJ86!
GND stands for Graduated Neutral Density. These numbers, 0.6 and 0.9 denote how "dark" the dark parts are, 0.6 is brighter than 0.9, and 0.3 and the brightest of the lot.
They are measured in stops, that correspond to the EV values on your camera. A GND is not a "function" that can be found on camera, but rather a rectangular (or square) piece of resin (Call it plastic if you will) that contain the dark and a clear side.
Basically, a GND darkens the sky because of the limited dynamic range of the camera. Thus also allowing slower shutter speeds and certain effects that usually "enhances" the sky to suit the dynamic range of what we see in our eyes.
GNDs come in "hard" and "soft" setups. Hard, as the name implies, a straight horizontal change with little or no graduation from the dark to the clear side. It looks something like this:
http://www.fredmiranda.com/forum/ufiles/17/424417.jpg
Soft GNDs, as the name implies, have longer gradation from dark to the clear side. The Soft GND is useful for situations where irregularity of the background or horizon exists; ie. mountains and buildings. To use a hard GND will create a straight gradation across the scene and will not look right at all. A soft GND looks like this:
http://www.fredmiranda.com/forum/ufiles/18/424418.jpg
Hope this helps.
