Ive been using la awesome for years and it works great on a car wash brush and a little scrub its gone. only bad thing about it is when its colder than 60 it wont work unless the gutter or the brush has hot water on it.
If you dont know what la awesome is they sell it at most 1.00 stores and dollargeneral, family dollar and i think walmart sells it to, its 1.00 for spray bottle or 3.00 for half gallon!
works wonders with carpet cleaners and just as a general cleaner, been adding it to all my house washes for years.
back to the question
what is everyone using for cleaning those streaks on gutters?