How different is a GND filter compared to a polarizing filter?
Check out one of Cokins systems.
Did you happen to try the search feature there at
B&H Photo Video, the link I provided for you? Did you happen to click on the feature tab that explains how the GND works. It's a good resource that I use several times a day for technical information as well as being one of the most reliable photo gear online shops in the US.
Go to
www.Wikipedia.org and learn what polarized light is and how a polarizing filter works. While you're there you can also check out:
Graduated Neutral Density filters, DOF, Aperture, Shutter Speed, How auto focus works, Exposure compensation, What is RAW, What is JPEG, What is diffraction and on and on.
A GND is a piece of optically clear glass that has about 1/2 of it darkened. You cover the brightest portion of a scene with the darkened portion while leavin gthe dimmer foreground unmodified. It's graduated from very dark at the top and fades to clear near the middle.
A polarizer is the same darkness from top to bottom.
BUT, it's polarizing effect is variable relative to the position of the Sun or the direction of the polarizing light that is hitting it. You rotate a polarizeer as you look through the viewfinder to adjust the effect. A polarizer works best when the Sun is below 30 degree's above the horizon and is 90 degrees to the lens axis.