You can get F-13 from Pressure Tek www.pressuretek.com
I use it for gutter whitening usually starting out with a weaker ratio of 8:1 water and F-13. If the gutters are more stubborn I'll bump up the mix a little bit.
I also use it in my custom housewash (power washing) mix which is called "Cherry Bomb". Cherry bomb is a housewash mix consisting of bleach, 4 oz. of F-13, 10 oz. of Simple Cherry house wash, and a shot of Dawn mixed with water in a 5 gallon container.
The F-13 gives the housewash mix a little more kick or cleaning power.
From what I understand, F-13 is more concentrate than Gutter Zap so one gallon of F-13 goes farther or lasts longer. Therefore the shipping cost of F-13 is more cost effective.