All setters were interbred back in the early 1800's.   There are documented pedigree's from that time showing Irish Setters behind Gordon Setter pedigrees.  The same must have worked in reverse as well because when an Irish is bred to almost anything else-the result is a black dog.  Gordon Setters were allowed to be shown in various colors until about 1905, when the Black and Tan pattern became recognized as the only acceptable color variation.

However, mother nature doesn't recognize human restrictions and continues to occasionally show up. Off color Gordon Setters may be registered, and can compete in all AKC events except for the conformation ring.   They can even be used for breeding purposes, but that is not reccomended.  The "standard" calls for a dog that is black with tan markings.   Breeding dogs of different colors only results in perpetuating the problem in generations to come.

As a side note: It is said by some that Golden Retrievers were developed out of Flat Coat Retrievers, and that Flat Coats were developed from Gordon Setters.   Could the Golden Color of the Golden Retriever actually be from "Red" Gordons?