So who were those colonial Americans that were fighting the Red Coats in an ungentlemanly manner - behind trees, fences, and other forms of cover?
Those were colonial militias. Gates used similar tactics in the Saratoga Campaign. Militias did fight effectively in the revolution. In addition, Francis Marion is usually credited with being the father of modern guerilla warfare.
However, that is not to say, “George Washington fought a guerilla war against the Red Coats”. He did not. His letters to congress consistently denigrate (unfairly, IMHO) militias. He was a conventional general with a conventional army, fighting a conventional war.