get yourself a
nylex car wash brush. You want to make sure it is soft and absorbent. The stiffer brushes won't conform to the irregular shape of the gutter as well. Get one with long soft bristles. I would just buy the head if you can so that you can attach it to a long extension pole. The one I got came with a 5 foot flow through handle that you can attach to a garden hose, but I just took it off and attached it to a long extension pole.
here's another example
truck & bus wash brush
Kitsap Roof & Gutter Cleaning