Further research has shown that there were indeed, many forms of plainchant, likely pre-Christian in origin, which antedate the so-called "Gregorian".
These include the aforementioned Gallican, but also "Old Roman" (which was suppressed in the 13th.-century), "Ambrosian", "Mozarabic" and "Beneventan".
"As a matter of fact, "Gregorian" music only dates from the year 593; and it was the Gallic (some say Ambrosian) chant which St. Patrick taught. Even assuming that the plain song of St. Gregory reached Ireland about the year 620, which is improbable, Irish psalmody and hymnody were distinctly Celtic in the first half of the seventh century, and were mainly "adaptations" of the old Irish pre-Christian melodies." (1)
There are fragmentary- but indicative examples, related to the underpinnings of a formerly chanted musical tradition. In the "Encyclopedia of Catholicism" we find, "There is evidence that a Celtic form of the liturgy and chant preceded the imposition of Roman usages by Augustine of Canterbury (d.604). No notation has survived. The Antiphonary of Bangor and the Irish Liber hymnorum contain early hymns that call for antiphonal singing, but no indications of what the early music would be. A few antiphons, unlike Roman types from Caen, are reconstructible from 13th-century manuscripts, including the chant Ibunt sancti, which the Irish monk Theobald of Bobbio was said to recite on his deathbead." (2)
As to the statement that "no notation has survived" I'm compelled to add, "that we know of." Since the Vatican's own archives are seemingly beyond external investigation, a possible source of documentary evidence (I'm speaking here of actual vellum from early monastic communities) may perhaps be found much closer to home. I think it likely that sacerdotal, missionary and contemplative Orders brought such heirlooms over from Ireland, Brittany and other European communities to newly created, North American foundations, but this remains speculative, on my part).
From the entry on Bangor, Ireland, founded by Comgall in 558 C.E. : "The Antiphonary of Bangor was written here between 680 and 691; it is the only surviving liturgical authority for the choir office of the early Irish Church". (3)
"Whatever Oriental features may be found in the early Irish Divine Office must have passed into it through a Gallican channel". The connecting link between Ireland and the East was Gaul".(4)
Basically, the point being made here is that there were many forms of chant used by early Christians in general; their roots do predate Roman Catholicism and were at variance with prescribed, papal custom. Whether they originated from Druidical sources in Gaul and Ireland is still debatable, but what seems apparent is that Eastern anchoritic practices, along with a chant-form of ancient precedence, were clearly preferred by the original followers of Celtic Christianity.
"Most of the Druids who had adopted the Christian religion had become filid, and though this class of poets was supervised by the church it formed an important enough elite to have considerable influence on ecclesiastical decisions, and to impose its own almost heretical brand of theology". (5)
Above: "Earliest Western notation for chant appears in the 9th century. These early staffless neumes, called cheironomic or in campo aperto, appeared as freeform wavy lines above the text. Various scholars see these as deriving from cheironomic hand-gestures, from the ekphonetic notation of Byzantine chant, or from punctuation or accent marks. A single neume could represent a single pitch, or a series of pitches all sung on the same syllable. Cheironomic neumes indicated changes in pitch and duration within each syllable, but did not attempt to specify the pitches of individual notes, the intervals between pitches within a neume, nor the relative starting pitches of different syllables' neumes." (6)
1: A History of Irish Music by William H. Grattan Flood.
www.libraryireland.com/IrishMusic/I.php2: Encyclopedia of Catholicism; By Frank K. Flinn, J. Gordon Melton pg. 145.
books.google.com/books3: The Concise Oxford Dictionary of the Christian Church | 2000 | E. A. Livingstone.
www.encyclopedia.com/topic/Bangor_%28Wales%29.aspx4: The Antiphonary of Bangor; William Griggs, Biblioteca ambrosiana.
books.google.com/books5: The Celts by Jean Markale; pg.145.
6: Neume Map;
maps.thefullwiki.org/Neume