The translation of the Gallic faith into the Roman pantheon
The continental neighbors of the Romans, the Gallic tribes were considered barbaric entities which the Republic and Empire attempted to colonize multiple times. Stretching through modern day France and Spain, the Romans came into contact with the Gauls consistently throughout their history, most prominently when Julius Caesar made it his mission to dominate the tribes on the coast of the English Channel. In doing so, he paved the way for two marches on the British Isles, most notably his infamous "crossing the Rubicon," though both times he failed to conquer the Insular Gauls. However, his numerous conquests on the mainland allowed for constant military encampment within the Gallic lands, resulting in a need to bring the Gallic religion under some kind of Roman control. This culminated in what is now known as the Gallo-Roman religion, an amalgamation of the two faiths.
The Vachères warrior, a statue of a Gaulish warrior wearing Roman clothing (ca. 1st century BC). Wikipedia, CC
With Caesar conquering much of Gaul during his Gallic Wars, the Roman military often made their home in various Gallic territories—both for the battles, and to keep the Roman power in place following their victories. Because of this, it is believed that the Roman soldiers needed a way to worship their own gods and goddesses in this new territory. One of the ways in which they accomplished this, also desiring to prevent overwhelming resistance from the native Gauls, was assimilation, wherein the Gallic gods were likened to the Roman gods. This act is known as translation.
A map of Gaul in the 1st century BC, showing the relative positions of its three tribes: Celtae (Galli), Belgae and Aquitani. The region corresponds to what is now Belgium, France, Switzerland, Netherlands, Western Germany and Northern Italy. Public Domain
It is important to understand that the gods of the Gallic religion were not the same as those of the Roman. The Romans believed, like the Greeks, that their gods were idealized humans—they not only took human shape but they participated in various forms of human interaction and experience. (That is to say that they loved, argued, took revenge, etc.) The Gallic gods, on the other hand, were representational deities—manifestations of the natural world. Not anthropomorphic, the springs and rivers and mountains and forests were worshipped as supernatural beings but did not to take on human form. Worship, therefore, took place at the specific locations, and there were few—if any—specific temples dedicated to these natural forces.
Apollo, Cernunos and Mercury. (public domain)
Gallic art reveals their belief in the gods quite clearly as, before the Romanization of the region, the gods were merely depicted as a consolidation of geometric shapes and stylized forms rather than bodily representations. Epona, for example, the goddess of horses in the Gallic faith, was often represented as a horse by the natives rather than as a woman. It was only when she was adopted by the Romans, one of the few deities taken from the Gauls and fully translated into the Roman pantheon, that she was depicted as a woman on a horse, riding into battle, alongside the Roman armies. Without the Roman influence, Epona would have remained a metaphor in art rather than a woman.
According to one of his written accounts, Caesar's Gallic Wars describes five primary gods of the Gallic religion. Their names, however, were those of five Roman gods: Mercury, Jupiter, Mars, Apollo, and Minerva. This was undoubtedly because the Romans associated the Gallic gods with their known Roman gods, believing—in a way—that all other pantheons were merely misnamed versions of their own. With their legions spread throughout the Gallic lands, and desiring to worship their native gods anyway, it was not all that difficult to associate the two faiths and thus retitle the Gallic deities. Adding a Roman epithet to the Gallic name allowed the two faiths to blend in such a way that the Gauls could still refer to their own gods while venerating those of Rome. This move was then followed by artistic integration, similar to the Roman adoption of Epona.
Epona, a resulting goddess from the Gallo-Roman fusion, was "the sole Celtic divinity ultimately worshipped in Rome itself." Epona and her horses, from Köngen, Germany, About 200 AD. Wikipedia, CC
The Celtic gods soon began to take on human forms, forms similar to the depiction of their Roman counterparts in the empire. There is no known definite iconography that the Gauls had for their gods so transforming the metaphorical images was not very difficult. Lugh, the god of light, soon came to look like Mercury; the protector Nodens began to hold the sword and helmet of Mars; Slius became known for armor that looked eerily similar to Minerva's, and so on. The five "primary" Gallic gods became very Roman in their appearance, thereby allowing the Gauls to continue to worship their deities in a Roman guise. This anthropomorphism was furthered by the Romans coupling the Gallic and Roman gods, creating intercultural relationships to reflect what was happening among the humans. Roman gods were given Gallic wives in the native regions, furthering cementing in the minds of the Gauls that the Romans were there to stay.