You'll probably need to clean them out by hand the first time. Use a small trowel, or a plastice cup....something along those lines.
I have a pressure washer with a long wand....then a U shaped accessory that sprays downward. I can stand on the ground and wash out the gutters.
This is a task that really needs to be done quarterly IMO. Get them clean, then keep them clean.
For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.