“Omnia Gallia in partes tres divisa est” - so said Gaius Ivlivs Caesar, by this meaning that all Gaul was divided in three parts. These parts have been, respectively, Gallia Cisalpina, or Gaul "behind" or "this side" of the Alps, Gallia "Transalpina" or "Provincia Nostra", which extended from the Alps to the modern day Auvergne in France, and Gallia "Comata" or "Hairy" Gaul.
This administrative division was adopted given a realistic evaluation of the conditions necessary towards their administration. While Gallia Cisalpina and Transalpina were well Romanized, thanks to early conquest and/or the establishment of colonies like Narbo Martius in modern-day Languedoc, most of the rest of Gaul remained a wild, foreign territory with traditional Celtic culture and usages. Therefore it was named "hairy" Gaul, perhaps a well known reference to Celtic customs. Of this, we have plenty of references: GALLIA INQUIT TOGATAM REMITTO, COMATAM POSTULO (CICERO, THE PHILLIPICS), GALLIA OMNIS COMATA [long final a] UNO NOMINE APPELATA (PLINY, NATURAL HISTORY).
All of these are explicit references to the least Romanized, further Gaul conquered during the Gallic Wars (58 to 51 BC), taken by Gaius Ivlivs Caesar during his famous campaign later described in De Bello Gallico. While Caesar himself never used that term, preferring the well known "Provincia Nostra" (also used to refer to Gallia Transalpina), the terminology became standard after the conquest. This terminology reflects the Roman view of further Gaul as a savage, barbarous hinterland, with clearly demeaning connotations. But we know with plenty of detail that the Gauls of Caesar's time were not barbarians dwelling in huts in the forest, as convinced as we may be by the skewed Roman point of view, and that the late conquest of further Gaul made it conserve its Celtic customs and usages for far longer than the areas named as "Provincia Nostra" or "Gallia Togata", which had by then been partially or fully Romanized. These customs had always been, due to an intense and long historical rivalry, seen by the Romans as barbarous and foreign, and in need of taming.
Historically, we know that the Romanization of Gaul was a gradual process which lasted for centuries. Archaeological evidences shows Celtic customs, calendars and feasts being held well into the 2nd century AD. Gallic tribes that had been allied to Rome were treated leniently, and conserved significant local autonomy, with many of their representatives even being later admitted into the Imperial Roman Senate. The Gallic language itself is attested, together with religious customs, well into the 5th Century AD - a Gallic Shrine, named as "Vasso Galatae", is attested as being razed by Christians during this later date. Nonetheless, as the centuries passed, gradual romanization quickly extinguished the old Celtic culture, mores and language, giving place to a Romanized environment and a local Latin dialect with significant residues of ancient Gaulish. This was to form the source of later, Medieval, Frankish culture which adopted the existing Roman law and culture, as well as the Gallo-Romance language.