The following stray lines may once have made (or could make) a single stanza.

Mellinger Edward Henry, Folk Songs form the Southern Highlands (N.Y.: J. J. Augustin, 1938). p. 333, coll. in N.C.:

James A. Garfield,
He took me to be his friend,
I shot a bullet into him
That caused his fatal end.


Tennessee Folklore Society Bulletin (Dec., 1953), p. 93, coll. in Tenn.:

The name of my revolver
Was the British Forty-Four
And when the bullet left the hull
It made an awful roar.


The point of the ballad, as usual, is the emotion: the shock and violence of the crime, the judgment of Providence, the tragedy of Guiteau's family, Guiteau's lack of remorse, and the horror of the execution. Factual accuracy need only be minimimal.

The song came to mind because Netflix is currenty showing a four-part miniseries abut Guiteau and Garfield called "Death by Lightning." I don't know if the ballad appears.

The series is described as "darkly humorous." Unlike the ballad. Count me out.