Who are the minor characters in the Great Gatsby and why?

Well, first we have to look at who the main characters are
-- Gatsby, Nick, Daisy, and Tom.  Since a minor character would most likely be described
as one who has an effect on the actions or emotions of the main characters then we can
say that the three most important minor characters are Jordan Baker and Myrtle &
George Wilson.  Jordan has a major impact on Nick, particularly in terms of his feelings
and the information that he acquires throughout the novel.  If it had not been for
Jordan, then Nick would not have known much of anything about Gatsby and Daisy's past. 
Myrtle is obviously important because of the role that she plays as Tom's mistress, and
it was her death that indirectly got Gatsby killed by her husband George.  The character
of Owl Eyes is also a minor character because he represents the same idea as the
billboard in that he sees everything that is happening -- which is recognized the last
time we see this charcter at Gatsby's funeral.  Other minor characters include
Klipspringer, the McKees, Katherine, and Michaleis.