Below are miscellaneous quotations dealing with the definition of "gospel song"/"gospel hymn". These are materials I gathered in trying to understand what others are saying about "gospel songs" and how they define them. There is a wide variety of usage of the terms.
"In comparison with hymns, which are generally of a statelier measure, the gospel song is expected to have a refrain and often a more syncopated rhythm." -- Wikipedia
"The folk hymn is known by its musical character. The melody, and it is usually assigned to the tenor, is often in one of the ancient modal scales. Certain tones are omitted or less conspicuously employed, giving the impression of a gapped scale of five or six notes." -- American Hymns Old and New, Vol. 2. Albert Christ-Janer. (New York: Columbia University Press, 1980, p. 293)
"The gospel hymn was developed to meet the needs of revival and prayer meetings...The mood of the text might be optimistic or pleading; the music was tuneful and easy to grasp. The rudimentary harmonies, the use of the chorus, the varied metric schemes, and the motor rhythms were characteristic. A marchlike movement as in 'Shall We Gather at the River' was especially typical. The device of letting the lower parts echo rhythmically a motive announced by the sopranos became a mannerism which was abused by later writers...The best of the gospel hymns have a direct simplicity which has appealed to singers ever since the appearance of the first gospel hymnals." (He places George F. Root among the earlier composers of the style.) -- American Hymns Old and New, Vol. 2. Albert Christ-Janer. (New York: Columbia University Press, 1980, pp. 365-66)
Gospel songs revolve around the seminal three chords: the I, IV, and V chords. The first phrase very often ends on the V chord and the second phrase resolves back to the I-chord. -- Pierce Phillips
In close harmony, the alto and tenor parts largely parallel the melody so that all three parts may be played on a keyboard, while the bass part, though not melodically tied to the soprano, fulfills a harmonic function. This describes the hymn tunes of Mason, Hastings, Bradbury, etc. as well as much early gospel music, especially of the more homophonic variety. -- paraphrase of Warren Steel
"Variety notwithstanding, by the end of the twentieth century there were three main streams of gospel music stylistically, each with various subsets. The oldest is usually called gospel song or gospel hymnody. By the twenty-first century, it might be called traditional gospel, or, more properly, historic gospel, for from it flow the other two streams of black gospel and Southern gospel.
"Traditional gospel grew out of the Northern, urban revival tradition of evangelist Dwight L. Moody and his songleader/soloist Ira D. Sankey in the 1870s, and it remained the dominant musical style of revivalist-oriented churches for more than a century. Its rhythmic, melodic and harmonic structures were rooted in the European tradition of music composition and performance....The music was melodically tuneful, employing eighth notes more often than the slower-feeling quarters. Compound meters, particularly 6/8, were characteristic, producing a lilting quality for which gospel hymnody became famous (as in, for example, "Blessed Assurance, Jesus is Mine"). The melodic range was designed for congregational song and was therefore limited to that of the untrained voice from about middle C to top-line F. Harmonies were generally primary triads, although secondary triads, borrowed minor chords, and secondary dominants became part of the harmonic vocabulary. The often-published characterization of gospel music employing 'barbershop' harmonies is inaccurate, however.
"Most characteristic of the gospel song was a contagious chorus or refrain that summed up the text's meaning in a succinct and memorable manner. The opening words of the refrain were usually the name of the song, unlike older hymns that were identifed by their opening words. The precedent for these choruses was the secular 'household' or 'parlor' song, composed by Stephen Foster and others. In fact, many of the first generation gospel hymnists such as George F. Root were successful composers of secular music in the verse/chorus mode. p. 293
"Aldine S. Kieffer was perhaps the single most important figure in the birth of Southern gospel music." p. 215, Encyclopedia of American gospel music by W. K. McNeil
"Gospel as a separate musical art form emerged primarily in the South and, as one music historian has argued, stands alongside jazz, blues, and country music as 'the fourth great genre of grass roots music' and 'the fourth major type of southern music'." p. 4, Close harmony: a history of southern gospel by James R. Goff
"12. The 'Gospel' Hymn...It came at last into world prominence with the work of the Evangelist Moody and his musical colleague, Ira D. Sankey, which was actively pursued in the United States and Britain during the 1870s, 1880s, and 1890s...widely circulated [in] Songs and Solos...drawing largely on a type of tune that had already become popular in the United States, the type with a lively rhythm and a harmonization consisting of little more than alternations of the three chief chords of the key (tonic, dominant, and subdominant). To this type the American W. B. Bradbury (1816-1868) had been a considerable contributor...When its history comes to be studied it will probably be found that behind Sankey and Moody lies the powerful influence of the American Camp Meeting." -- p. 504, The Oxford Companion to Music by Percy A. Scholes (10th edition revised and edited by John Owen Ward), London: Oxford University Press, 1975
"Musically, the typical gospel song is in a major key, in common (4/4) time, with numerous repeated notes in a melody that is more interesting than the parts that accompany it." p. 288 "Gospel songs of this type often display repetitive rhythmic and melodic features and rudimentary antiphonal and responsorial textures." -- p. 300, A Portion for the Singers, R. Paul Drummond, Christian Baptist Press, 1989
"The gospel song seems to embody all that the old Sacred Harp songs did not: close harmony, the use of accidentals, and in some cases the concentration of melodic interest into a single part." -- p. 153, Public Worship, Private Faith, John Bealle