GND is very useful to help balance out the exposure since the sky will usually be a lot brighter than the ground/surrounds. CPL helps to make the colours deeper. And yes, u can have a normal ring-type CPL on ur lens and then a cokin (or equiv) holder mounted on that with a GND and maybe an ND or 2 if you want to go for the long exposure, smokey water look. This way, u have full control over where the gradient ends up. However, don't try this on a crop-factor wide angle lens. It will likely vignette a lot. A FF lens on a crop body works well tho.
Normally I will look out for mid tone within the frame. The moment when the sun almost set at the horizon ( I think it only last for 1 min ), the sun is not bright and glaring. I will meter the sky next to the sun and compensate accordingly.
Taking SUnset/Sunrise shots are really fun. Its different everyday. SO the better hit rate would be thorgh more shooting.
Here are some of my tips.
1) Use and observe nature: If you are planning to shoot sunset, you can monitor horizon cloud or atmospheric behaviours of the sunrise of tha same day. From my experience, the sun set of the day should 70% look lke the sun rise of the day as the atmospheric content of the area within the same day may still be the same a the end of the day. This curtain of atmospheric particles will effect the color, intensity and dtals of the clouds. Sad to say, st times a polluted sky can give you good color and also attenuate (cut-down) the brightness of the sun setting and hence allow better details. As for the sun rise, watch the sunset, it should be quite similar. All these greatly deopending on the cloud cover too!!! When ther is clouds......try t use it to addd details...like this one..
Effects of pollution.....no need ND filter.....
FYI: Do you know that the scientists identify what gasses there are in the Sun by means of shining a beam of light in to a chamber filled with different gasses. With the different gasses, the colors of the light beam changes.
2) Balancing exposure: Once you have a good dynamic range of subject, the you can use NG grad filters like Cokin/Singh Ray etc to cut the light analogous to using a sunglasses. If the intensity of the sun is too great, using too much filter may result in the other areas to be too dark. Get a general exposure of the scene with the desired foreground exposure. Take the Tv Av values and plug in into the camera in Manaul mode (assuming using SLR).
Then you can use say a ND2~ND8 Graduated or a mixture of them (coupling 4+2 etc.) to get the desired effect. Put on a shirt that have many pockets that will enable you to pull the right mix of NG filters. have the filters ready and flick the neccesary ND power as the sun sets. Make sure that you meter your scene first before puttin ghte ND filters on...
You can mix ND filters and also subjects to hide strong direct sun like this..