In The Song of the Lark, Thea sees the Dying Gladiator in the Chicago Art museum: "the Dying Gladiator she had read about in 'Childe Harold' almost as long ago as she could remember." He is described in Canto 4, stanza 140 of Childe Harold's Pilgrimage.
In Song of the Lark, Thea reads this "book of Byron's poems" as a young girl, and particularly likes "My native land, good-night" (Canto 1, stanza 13) and "There was a sound of revelry" (Canto 4, stanza 21).
In O Pioneers, Emil "seemed intent upon his own thoughts, and, like the Gladiator's, they were far away." The Dying Gladiator appears in Childe Harold's Pilgrimage, Canto 4, stanza 140.