John Gray (sometimes spelled Grey) was hired by the Englishman W. G. Romaine as a guide for his summer tour of the American West. Gray was half Scottish and half Iroquois, a St. Regis Mohawk, born around 1795 in upstate New York. His father, William L. Gray, had served as a soldier during the American Revolution and then married into the Mohawks of Akwesasne. John Gray also went by the Iroquois name of Ignace Hatchiorauquasha, St. Ignatius being his patron saint.

John Gray, as depicted by Nicholas Point, S.J. Picture from http://www.hunterbear.org/family_stuff.htm
Father Nicholas Point, one of the priests with Father DeSmet, drew this portrait of John Gray while on the trail. The smaller picture is of Gray’s wife.
The Catholic priests had a good opinion of Gray and his abilities. Father Gregory Mengarini records the following:
“So the sun rose and the sun set, and the end of our journey was still over a thousand miles away. Sometimes John Grey would say to me in the morning, “Father, so you see that speck in the distance? Today we must reach there.” “Then our day’s travel will be short,” I would answer. “We shall see,” he would say laughingly. And the hours of the morning would pass and we would be already journeying long under a scorching afternoon’s sun before that speck would achieve appreciable magnitude and distinctness of form.”
Gray entered the fur trade sometime around 1818, about the same time that he married his wife Marienne, also a Mohawk. He was active in the fur trade for the next 25 years or so.
Gray and Romaine, along with four other men, left the wagon train on July 25th to return to the United States. The trip with Romaine was probably the last of Gray’s excursions to the West, after which he retired to his home in Kansas City, Missouri. He was killed in 1848 in a dispute with a neighbor.
For more about John Gray, consult The Mountain Men and the Fur Trade of the Far West, by Leroy R. Hafen, or see the website of his great-great-great grandson Hunter Gray.