He is best known for having been identified as one of the six Marines who helped raise the second of two U.S.
flags atop Mount Suribachi on February 23, 1945, in the iconic photograph Raising the Flag on Iwo Jima.
On October 16, 2019, the Marine Corps stated that Private First Class Harold Keller was actually depicted in the photograph, not Gagnon.In 1954, the Marine Corps War Memorial in Arlington, Virginia, was modeled after the famous photograph of the second flag raising which had been generally portrayed since 1945 as the only flag-raising on Mount Suribachi.
Although there were photographs taken of the first flag flying on Mount Suribachi, there was no single photograph taken of Marines actually raising the first flag.
The first flag flown over the southern end of Iwo Jima was regarded to be too small to be seen by all the American troops on the other side of the mountain, and was replaced hours later the same day by a larger one which had been carried up the mountain by Gagnon.
The replacement flag went up as the first flag was being taken down.