The Roman conquest caused the Gallic customs to disappear, i.e. the conquered race absorbed the conquering race.
The Gauls and the Romans were indeed of the same stock, the Aryan stock. This community of origin, combined with the prestige of Roman civilisation, explains the rapidity of assimilation.
By changing their ideas, the conquered changed their way of life, and the Gallo-Roman cities no longer resembled the Gallic villages.
In fact, there is no art, there is no Gallo-Roman architecture. It is limited to imitating the masters from beyond the Alps.
The type we see at the Champ de Mars under the name of Gallo-Roman seems to belong to this transitional period, when Roman art is falling into decadence and Romanesque art is not yet fully established; the use of the semicircular arch appears.
© L'exposition de Paris - 1889