Has your pressure washer got adjustable pressure?
If so, turn it right down and use the black detergent nozzle (assuming you have the coloured quick release nozzles) for cleaning out the gutters. You don't need a lot of pressure.
If you don't have adjustable pressure, then sorry don't know the answer.
Obviously always work away from the neighbours on a semi.
On a semi always worth asking the neighbours if they want their's doing whilst you're there, or get your customer to ask them when you quote and offer both a discount. Even if they don't want their's done I would knock their door and explain what your doing and that you will clean up any mess when your finished.
Neil