The deck cleaning solution I use works pretty well when sprayed on gutters, but works best if you can then rub the gutter with a brush or cloth.

1/3 cup powdered laundry soap
2/3 cup Tri Sodium Phosphate
1 quart bleach
3 quarts hot water

Spray it on using a gallon pump sprayer, and then leave it on for 15 minutes, reapplying as needed to keep the surface wet or misting with water. Rinse thoroughly.

I have seen plastic rims that you place over the outer edge of the gutter that supposedly stops the trickle-down stains. Don't know if it works or not.

StainGoner is the best. I think its formula is patent pending. I used it and it started working before I got to wipe off the stains. I bought it from stainguarder.com.

Here is a link that might be useful: stainguarder