The other guys on the window cleaning forum I frequent use gutter zap, gutter grenade f13, purple power, or simple green. I have been using simple green because it is easy to find and it is concentrated. Although it doesn't get very cold in Western Washington so I don't know how it performs in the cold. I used to use bleach, which works great, but the problem was that it can drip onto decks and bleach the wood. I guess it will works if you have a helper ready with a hose to rinse the deck. Bleach remains effective in the cold I think. I use a car wash brush attached to an unger 30 foot extension pole.
Kitsap Window & Gutter Cleaning