Since no one else has chimed in, I'll do my best.
GND stands for Graduated Neutral Density. Mainly they are used in landscape photography to tame skys and prevent blown highlights. With the move to digital, they are to my mind redundant - better results can more easily be had by bracketing a shot and blending in photoshop either using masks or the HDR function.
A polarising filter on the other hand is seriously useful - they cut reflections and saturate colours. On a lens as wide as a 10-20 though, you need to be aware that you won't get an even effect. You'll need someone more scientific than me to explain why. Nonetheless, I often use one on my 10-20.