You're right that Roman Gaul underwent, trough romanisation and inclusion on Roman institutions and networks that swept clean most of pre-Roman Gaulish institutions and culture by the early middle-ages.
Romanization of the Gallic provinces is still best understood as a creolization, that is provincial culture was up to a point a mix of direct imperial influence and transformed indigenous practices on Roman terms: for example, a Gaul in the Ist century might well worship an ancient local deity in addition to the imperial religious set, but would do so on Roman fashion.

As such, romanization necessarily factored in Gallic clothing, either directly transmitting imperial vestment in the provinces or by influencing the make-up of local customs. Note that both weren't mutually exclusive : at the contrary, clothing as a marker of wealth, legal or social status as much as everyday practicability was more of a set one could pick depending of its needs. For instance, the toga was an high-status piece marking one as a citizen or public servant but of little use elsewhere and the same person bearing it could likely use a tunic whose cut would have been similar to a poorer citizen but made of quality textile.

Gallo-Roman clothing was thus a set borrowing as much from Imperial as local contemporary traditions : their tunics were longer as it happened in Italy but sawn rather than draped, adopting as much the Mediterranean palium than continuing to use the rainproof birrus, etc. We have ample evidence, trough artistic representations, literary sources and archaeological of these provincial vestment. These same sources still, show that the breeches and the woollen sagum, disappear from the provinces by the end of the Ist century CE when Gaul as a whole became firmly integrated into the Roman Empire, where elites looked after the imperial models and while not ceasing to consider themselves as Gauls, had entirely appropriated Roman practices.

In 100 CE, breeches are entirely gone. Well, not entirely... Although breeches did fall out of fashion in Gaul as a civilian clothing, Roman soldiers stationed in the northern borders adopted them as part of their own vestment, as it was customary of adopting military features or social codes from peoples they fought and recruited from. Either adopted from Germans (whose breeches were essentially similar to pre-Roman Gauls) or from Gaulish features creolized in the military crucible (as the worship of Epona, the tunic known as caraccala, the collar bear and mustache, etc.) and quite likely both, the breeches would thus survive as part of the Roman military culture before coming back in force in the IIIrd century in most of the western Empire (except in Italy where not only its "barbarity" but also its identification as military, made it suspect and unpalatable for civilian authorities) both the late imperial Roman culture and the Barbarian social codes following suit.