The words of the song are based on a traditional French carol known as Les Anges dans nos Campagnes (literally, The Angels in our Countryside). Its most common English version was translated in 1862 by James Chadwick.
where the sung vowel sound "o" of "Gloria" is fluidly sustained through a lengthy rising and falling melismatic melodic sequence:
"Gloria in Excelsis Deo" is itself the name of an older famous hymn.
The phrase also appears melismatically in the Latin version of the carol "O Come All Ye Faithful", though somewhat less extended:
In England, the words of James Montgomery's "Angels from the Realms of Glory" are sung to this tune, except with the "Gloria in excelsis Deo" refrain.
In the English version of "O Come All Ye Faithful", that phrase is poetically translated as Glo-ry to Go-od, Glo-ry in the High-est, reducing the melisma to no more than two notes per word.