Si (Cy) or Silas Gordon was a citizen of Platte City, Missouri. He was very active in opposing the Union 18th Missouri Infantry and incursions by Kansas pro-Union soldiers into Buchanon, Clay and Platte Counties in early 1861. Si Gordon raised a partisan band and took the field against what he considered as an illegal invasion of his home territory. He was accused of several actions in the area including the burning of the Platte River Bridge near St. Joseph, Missouri in September 1861. He was finally forced away from the area and commenced operations either in conjunction with or under William Quantrell.
The book mentioned before gives a better sense of and detail of Gordon's known activities.
Hope this helps.